title: How to install pg in a rails app?
body: <p>When I include </p>
  
  <pre><code>group :production do
    gem 'pg'
  end
  </code></pre>
  
  <p>in my gemfile and run 'bundle install' I get an error saying:</p>
  
  <pre><code>Gem::Installer::ExtensionBuildError: ERROR: Failed to build gem native extension.
  
              /Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/bin/ruby extconf.rb 
      checking for pg_config... yes
      Using config values from /usr/bin/pg_config
      checking for libpq-fe.h... yes
      checking for libpq/libpq-fs.h... yes
      checking for pg_config_manual.h... yes
      checking for PQconnectdb() in -lpq... yes
      checking for PQconnectionUsedPassword()... yes
      checking for PQisthreadsafe()... yes
      checking for PQprepare()... yes
      checking for PQexecParams()... yes
      checking for PQescapeString()... yes
      checking for PQescapeStringConn()... yes
      checking for PQescapeLiteral()... yes
      checking for PQescapeIdentifier()... yes
      checking for PQgetCancel()... yes
      checking for lo_create()... yes
      checking for pg_encoding_to_char()... yes
      checking for pg_char_to_encoding()... yes
      checking for PQsetClientEncoding()... yes
      checking for PQlibVersion()... yes
      checking for PQping()... yes
      checking for PQsetSingleRowMode()... no
      checking for rb_encdb_alias()... yes
      checking for rb_enc_alias()... yes
      checking for rb_thread_call_without_gvl()... yes
      checking for rb_thread_call_with_gvl()... yes
      checking for rb_thread_fd_select()... yes
      checking for rb_w32_wrap_io_handle()... no
      checking for PGRES_COPY_BOTH in libpq-fe.h... no
      checking for PGRES_SINGLE_TUPLE in libpq-fe.h... no
      checking for struct pgNotify.extra in libpq-fe.h... yes
      checking for unistd.h... yes
      checking for ruby/st.h... yes
      creating extconf.h
      creating Makefile
  
      make
      gcc-4.2 -I. -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/x86_64-darwin12.2.1 -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/ruby/backward -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1 -I. -DRUBY_EXTCONF_H=\"extconf.h\" -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -I/usr/include -fno-common -O3 -I/opt/sm/pkg/active/include -fno-common -pipe  -o gvl_wrappers.o -c gvl_wrappers.c
      gcc-4.2 -I. -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/x86_64-darwin12.2.1 -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/ruby/backward -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1 -I. -DRUBY_EXTCONF_H=\"extconf.h\" -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -I/usr/include -fno-common -O3 -I/opt/sm/pkg/active/include -fno-common -pipe  -o pg.o -c pg.c
      pg.c: In function ‘Init_pg_ext’:
      pg.c:384: error: ‘PQPING_OK’ undeclared (first use in this function)
      pg.c:384: error: (Each undeclared identifier is reported only once
      pg.c:384: error: for each function it appears in.)
      pg.c:386: error: ‘PQPING_REJECT’ undeclared (first use in this function)
      pg.c:388: error: ‘PQPING_NO_RESPONSE’ undeclared (first use in this function)
      pg.c:390: error: ‘PQPING_NO_ATTEMPT’ undeclared (first use in this function)
      make: *** [pg.o] Error 1
  
  
      Gem files will remain installed in /Users/sharatakasapu/.bundler/tmp/36975/gems/pg-0.15.1 for inspection.
      Results logged to /Users/sharatakasapu/.bundler/tmp/36975/gems/pg-0.15.1/ext/gem_make.out
  
      An error occurred while installing pg (0.15.1), and Bundler cannot continue.
      Make sure that `gem install pg -v '0.15.1'` succeeds before bundling.
  </code></pre>
  
  <p>When I do:</p>
  
  <pre><code>Sharats-MacBook-Pro:sentiment sharatakasapu$ which pg_config
      /usr/bin/pg_config
      Sharats-MacBook-Pro:sentiment sharatakasapu$ gem install pg -- --with-pg-config= '/usr/bin/pg_config'
      Building native extensions.  This could take a while...
      ERROR:  Error installing pg:
          ERROR: Failed to build gem native extension.
  
              /Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/bin/ruby extconf.rb --with-pg-config= /usr/bin/pg_config
      Using config values from 
      sh: : command not found
      sh: : command not found
      checking for libpq-fe.h... yes
      checking for libpq/libpq-fs.h... yes
      checking for pg_config_manual.h... yes
      checking for PQconnectdb() in -lpq... yes
      checking for PQconnectionUsedPassword()... yes
      checking for PQisthreadsafe()... yes
      checking for PQprepare()... yes
      checking for PQexecParams()... yes
      checking for PQescapeString()... yes
      checking for PQescapeStringConn()... yes
      checking for PQescapeLiteral()... yes
      checking for PQescapeIdentifier()... yes
      checking for PQgetCancel()... yes
      checking for lo_create()... yes
      checking for pg_encoding_to_char()... yes
      checking for pg_char_to_encoding()... yes
      checking for PQsetClientEncoding()... yes
      checking for PQlibVersion()... yes
      checking for PQping()... yes
      checking for PQsetSingleRowMode()... no
      checking for rb_encdb_alias()... yes
      checking for rb_enc_alias()... yes
      checking for rb_thread_call_without_gvl()... yes
      checking for rb_thread_call_with_gvl()... yes
      checking for rb_thread_fd_select()... yes
      checking for rb_w32_wrap_io_handle()... no
      checking for PGRES_COPY_BOTH in libpq-fe.h... no
      checking for PGRES_SINGLE_TUPLE in libpq-fe.h... no
      checking for struct pgNotify.extra in libpq-fe.h... yes
      checking for unistd.h... yes
      checking for ruby/st.h... yes
      creating extconf.h
      creating Makefile
  
      make
      gcc-4.2 -I. -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/x86_64-darwin12.2.1 -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/ruby/backward -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1 -I. -DRUBY_EXTCONF_H=\"extconf.h\" -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -I -fno-common -O3 -I/opt/sm/pkg/active/include -fno-common -pipe  -o gvl_wrappers.o -c gvl_wrappers.c
      gcc-4.2 -I. -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/x86_64-darwin12.2.1 -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1/ruby/backward -I/Users/sharatakasapu/.rvm/rubies/ruby-1.9.2-p320/include/ruby-1.9.1 -I. -DRUBY_EXTCONF_H=\"extconf.h\" -D_XOPEN_SOURCE -D_DARWIN_C_SOURCE   -I -fno-common -O3 -I/opt/sm/pkg/active/include -fno-common -pipe  -o pg.o -c pg.c
      pg.c: In function ‘Init_pg_ext’:
      pg.c:384: error: ‘PQPING_OK’ undeclared (first use in this function)
      pg.c:384: error: (Each undeclared identifier is reported only once
      pg.c:384: error: for each function it appears in.)
      pg.c:386: error: ‘PQPING_REJECT’ undeclared (first use in this function)
      pg.c:388: error: ‘PQPING_NO_RESPONSE’ undeclared (first use in this function)
      pg.c:390: error: ‘PQPING_NO_ATTEMPT’ undeclared (first use in this function)
      make: *** [pg.o] Error 1
  </code></pre>
  
  <p>I've been going through answers on SO but am having little success to fix this error.  Any advice?</p>