title: How to install R packages that use header files (RJSONIO, Rcpp)?
body: <p>I'm unable to install several R packages because they always fail on the first include statement. This is the error I get for Rcpp:</p>
  
  <pre><code>install.packages("/Users/nacho/Downloads/Rcpp_0.10.3.tar.gz", repos=NULL, type="source")
  Installing package(s) into ‘/Users/nacho/Library/R/2.15/library’
  (as ‘lib’ is unspecified)
  * installing *source* package ‘Rcpp’ ...
  ** package ‘Rcpp’ successfully unpacked and MD5 sums checked
  ** libs
  c++ -I/usr/local/Cellar/r/2.15.2/R.framework/Resources/include -DNDEBUG -fopenmp -I/usr/local/include   -fopenmp -fPIC  -g -O2  -c Date.cpp -o Date.o
  clang: warning: argument unused during compilation: '-fopenmp'
  clang: warning: argument unused during compilation: '-fopenmp'
  Date.cpp:29:10: fatal error: 'Rcpp/Datetime.h' file not found
  #include &lt;Rcpp/Datetime.h&gt;
           ^
  1 error generated.
  make: *** [Date.o] Error 1
  ERROR: compilation failed for package ‘Rcpp’
  * removing ‘/Users/nacho/Library/R/2.15/library/Rcpp’
  Warning message:
  In install.packages("/Users/nacho/Downloads/Rcpp_0.10.3.tar.gz",  :
    installation of package ‘/Users/nacho/Downloads/Rcpp_0.10.3.tar.gz’ had non-zero exit status
  </code></pre>
  
  <p>I get the same error for RJSONIO</p>
  
  <pre><code>install.packages("/Users/nacho/Downloads/RJSONIO_1.0-3.tar.gz", repos=NULL, type="source")
  Installing package(s) into ‘/Users/nacho/Library/R/2.15/library’
  (as ‘lib’ is unspecified)
  * installing *source* package ‘RJSONIO’ ...
  ** package ‘RJSONIO’ successfully unpacked and MD5 sums checked
  Trying to find libjson.h header file
  checking for gcc... gcc
  checking for C compiler default output file name... a.out
  checking whether the C compiler works... yes
  checking whether we are cross compiling... no
  checking for suffix of executables...
  checking for suffix of object files... o
  checking whether we are using the GNU C compiler... yes
  checking whether gcc accepts -g... yes
  checking for gcc option to accept ISO C89... none needed
  USE_LOCAL = ""
  Minor Version: 6
  Using local libjson code. Copying files from src/libjson/Source
  /private/var/folders/tl/_8_djcq15pl01ht8z6hy9tww0000gn/T/RtmpMKCuPF/R.INSTALL116ed562675dc/RJSONIO
  JSONAllocator.cpp
  JSONChildren.cpp
  JSONDebug.cpp
  JSONIterators.cpp
  JSONMemory.cpp
  JSONNode.cpp
  JSONNode_Mutex.cpp
  JSONPreparse.cpp
  JSONStream.cpp
  JSONValidator.cpp
  JSONWorker.cpp
  JSONWriter.cpp
  internalJSONNode.cpp
  libjson.cpp
  configure: creating ./config.status
  config.status: creating src/Makevars
  config.status: creating cleanup
  ** libs
  cc -I/usr/local/Cellar/r/2.15.2/R.framework/Resources/include -DNDEBUG -fopenmp -I/usr/local/include   -fopenmp -fPIC  -g -O2  -c ConvertUTF.c -o ConvertUTF.o
  clang: warning: argument unused during compilation: '-fopenmp'
  clang: warning: argument unused during compilation: '-fopenmp'
  c++ -I/usr/local/Cellar/r/2.15.2/R.framework/Resources/include -DNDEBUG -fopenmp -I/usr/local/include   -fopenmp -fPIC  -g -O2  -c JSONAllocator.cpp -o JSONAllocator.o
  clang: warning: argument unused during compilation: '-fopenmp'
  clang: warning: argument unused during compilation: '-fopenmp'
  JSONAllocator.cpp:1:10: fatal error: 'JSONAllocator.h' file not found
  #include "JSONAllocator.h"
         ^
  1 error generated.
  make: *** [JSONAllocator.o] Error 1
  ERROR: compilation failed for package ‘RJSONIO’
  * removing ‘/Users/nacho/Library/R/2.15/library/RJSONIO’
  Warning message:
  In install.packages("/Users/nacho/Downloads/RJSONIO_1.0-3.tar.gz",  :
  installation of package ‘/Users/nacho/Downloads/RJSONIO_1.0-3.tar.gz’ had non-zero exit status  
  </code></pre>
  
  <p>My sessionInfo()</p>
  
  <pre><code>R version 2.15.2 (2012-10-26)
  Platform: x86_64-apple-darwin11.4.2 (64-bit)
  
  locale:
  [1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
  
  attached base packages:
  [1] graphics  grDevices utils     datasets  stats     grid      methods   base
  
  other attached packages:
  [1] clickme_0.0.3   ggplot2_0.9.3.1 testthat_0.7.1  devtools_1.1
  
  loaded via a namespace (and not attached):
   [1] colorspace_1.2-1   dichromat_2.0-0    digest_0.6.3       evaluate_0.4.3     formatR_0.7        gtable_0.1.2
   [7] httr_0.2           knitr_1.1          labeling_0.1       MASS_7.3-23        memoise_0.1        munsell_0.4
  [13] parallel_2.15.2    plyr_1.8           proto_0.3-10       RColorBrewer_1.0-5 RCurl_1.95-4.1     reshape2_1.2.2
  [19] scales_0.2.3       stringr_0.6.2      tools_2.15.2       whisker_0.1        yaml_2.1.7
  </code></pre>