title: clang is using the wrong system include directory
body: <p>My workstation is RHEL 6.5, which came with gcc 4.4.7 preinstalled.  We are doing C++11 work, so we purchased the RedHat Developer's Toolset set of repositories, which includdes G++ 4.8.2.  Both versions of G++ are installed on this machine, but 4.8.2 is the version that is invoked when launched from the command line and via a build:</p>
  
  <pre><code>john@hurricane /home/john/repo/phoenix [PHX-29|✚5]  $ which g++
  /opt/rh/devtoolset-2/root/usr/bin/g++
  john@hurricane /home/john/repo/phoenix [PHX-29|✚5]  $ g++ --version
  g++ (GCC) 4.8.2 20140120 (Red Hat 4.8.2-15)
  Copyright (C) 2013 Free Software Foundation, Inc.
  This is free software; see the source for copying conditions.  There is NO
  warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
  </code></pre>
  
  <p>Note the unusual install location.</p>
  
  <p>I've also built clang from source and installed it.  However when I build C++11 projects using clang, clang is picking up the wrong system includes.  It's picking up the system headers from gcc 4.4.7, rather than from 4.8.2.</p>
  
  <p>G++ configuration:</p>
  
  <pre><code>john@hurricane /home/john/repo/phoenix [PHX-29|✚5]  $ g++ -E -x c++ - -v &lt; /dev/null 
  Using built-in specs.
  COLLECT_GCC=g++
  Target: x86_64-redhat-linux
  Configured with: ../configure --prefix=/opt/rh/devtoolset-2/root/usr --mandir=/opt/rh/devtoolset-2/root/usr/share/man --infodir=/opt/rh/devtoolset-2/root/usr/share/info --with-bugurl=http://bugzilla.redhat.com/bugzilla --enable-bootstrap --enable-shared --enable-threads=posix --enable-checking=release --with-system-zlib --enable-__cxa_atexit --disable-libunwind-exceptions --enable-gnu-unique-object --enable-linker-build-id --enable-languages=c,c++,fortran,lto --enable-plugin --with-linker-hash-style=gnu --enable-initfini-array --disable-libgcj --with-isl=/builddir/build/BUILD/gcc-4.8.2-20140120/obj-x86_64-redhat-linux/isl-install --with-cloog=/builddir/build/BUILD/gcc-4.8.2-20140120/obj-x86_64-redhat-linux/cloog-install --with-mpc=/builddir/build/BUILD/gcc-4.8.2-20140120/obj-x86_64-redhat-linux/mpc-install --with-tune=generic --with-arch_32=i686 --build=x86_64-redhat-linux
  Thread model: posix
  gcc version 4.8.2 20140120 (Red Hat 4.8.2-15) (GCC) 
  COLLECT_GCC_OPTIONS='-E' '-v' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
   /opt/rh/devtoolset-2/root/usr/libexec/gcc/x86_64-redhat-linux/4.8.2/cc1plus -E -quiet -v -D_GNU_SOURCE - -mtune=generic -march=x86-64
  ignoring nonexistent directory "/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include-fixed"
  ignoring nonexistent directory "/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../x86_64-redhat-linux/include"
  #include "..." search starts here:
  #include &lt;...&gt; search starts here:
   /opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2
   /opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/x86_64-redhat-linux
   /opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../include/c++/4.8.2/backward
   /opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/include
   /usr/local/include
   /opt/rh/devtoolset-2/root/usr/include
   /usr/include
  End of search list.
  # 1 "&lt;stdin&gt;"
  # 1 "&lt;command-line&gt;"
  # 1 "&lt;stdin&gt;"
  COMPILER_PATH=/opt/rh/devtoolset-2/root/usr/libexec/gcc/x86_64-redhat-linux/4.8.2/:/opt/rh/devtoolset-2/root/usr/libexec/gcc/x86_64-redhat-linux/4.8.2/:/opt/rh/devtoolset-2/root/usr/libexec/gcc/x86_64-redhat-linux/:/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/:/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/
  LIBRARY_PATH=/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/:/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../../lib64/:/lib/../lib64/:/usr/lib/../lib64/:/opt/rh/devtoolset-2/root/usr/lib/gcc/x86_64-redhat-linux/4.8.2/../../../:/lib/:/usr/lib/
  COLLECT_GCC_OPTIONS='-E' '-v' '-shared-libgcc' '-mtune=generic' '-march=x86-64'
  </code></pre>
  
  <p>clang++ configuration:</p>
  
  <pre><code>john@hurricane /home/john/repo/phoenix [PHX-29|✚5]  $ clang++ -E -x c++ - -v &lt; /dev/null 
  clang version 3.6.0 (trunk 222833) (llvm/trunk 222832)
  Target: x86_64-unknown-linux-gnu
  Thread model: posix
  Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.4
  Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.7
  Selected GCC installation: /usr/lib/gcc/x86_64-redhat-linux/4.4.7
  Candidate multilib: .;@m64
  Candidate multilib: 32;@m32
  Selected multilib: .;@m64
   "/usr/local/bin/clang" -cc1 -triple x86_64-unknown-linux-gnu -E -disable-free -disable-llvm-verifier -main-file-name - -mrelocation-model static -mthread-model posix -mdisable-fp-elim -fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables -target-cpu x86-64 -target-linker-version 2.23.52.0.1 -v -dwarf-column-info -resource-dir /usr/local/bin/../lib/clang/3.6.0 -internal-isystem /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7 -internal-isystem /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/x86_64-redhat-linux -internal-isystem /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/backward -internal-isystem /usr/local/include -internal-isystem /usr/local/bin/../lib/clang/3.6.0/include -internal-externc-isystem /include -internal-externc-isystem /usr/include -fdeprecated-macro -fdebug-compilation-dir /home/john/repo/phoenix -ferror-limit 19 -fmessage-length 271 -mstackrealign -fobjc-runtime=gcc -fcxx-exceptions -fexceptions -fdiagnostics-show-option -fcolor-diagnostics -o - -x c++ -
  clang -cc1 version 3.6.0 based upon LLVM 3.6.0svn default target x86_64-unknown-linux-gnu
  ignoring nonexistent directory "/include"
  #include "..." search starts here:
  #include &lt;...&gt; search starts here:
   /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7
   /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/x86_64-redhat-linux
   /usr/lib/gcc/x86_64-redhat-linux/4.4.7/../../../../include/c++/4.4.7/backward
   /usr/local/include
   /usr/local/bin/../lib/clang/3.6.0/include
   /usr/include
  End of search list.
  # 1 "&lt;stdin&gt;"
  # 1 "&lt;built-in&gt;" 1
  # 1 "&lt;built-in&gt;" 3
  # 320 "&lt;built-in&gt;" 3
  # 1 "&lt;command line&gt;" 1
  # 1 "&lt;built-in&gt;" 2
  # 1 "&lt;stdin&gt;" 2
  
  john@hurricane /home/john/repo/phoenix [PHX-29|✚5]  $ 
  </code></pre>
  
  <p>I'm sure this can be fixed by passing the right parameters to clang's <code>configure.sh</code>, but I'm not sure what to pass?</p>