title: Haskell package missing c library
body: <p>I'm having trouble building the <a href="https://github.com/albertoruiz/hmatrix" rel="nofollow">hmatrix library</a> on OS X Lion. Looking at the .cabal file, it requires the <code>gsl</code> library, so I installed it with macports. The .a files are in /opt/local/lib and the .h files are in /opt/local/include/gsl</p>
  
  <p>As suggested <a href="http://www.haskell.org/pipermail/beginners/2010-February/003456.html" rel="nofollow">here</a>  I changed the built-type from Custom to Simple.  (without that change I get a similar error).</p>
  
  <p>When I use <code>cabal configure</code> I get the following output:</p>
  
  <pre><code>* Missing C library: gsl
  This problem can usually be solved by installing the system package that
  provides this library (you may need the "-dev" version). If the library is
  already installed but in a non-standard location then you can use the flags
  --extra-include-dirs= and --extra-lib-dirs= to specify where it is.
  </code></pre>
  
  <p>So I tried <code>cabal --extra-include-dirs=/opt/local/include --extra-lib-dirs=/opt/local/lib configure</code>, but I still get the same error.  I can compile and link a c program that includes gsl.  What files is cabal looking for?  If I have the right files, how do I tell it how to find them?  </p>
  
  <p>libgsl.a is a universal binary:</p>
  
  <pre><code>$ file /opt/local/lib/libgsl.a
      /opt/local/lib/libgsl.a: Mach-O universal binary with 2 architectures
      /opt/local/lib/libgsl.a (for architecture x86_64):  current ar archive random library
      /opt/local/lib/libgsl.a (for architecture i386):    current ar archive random library
  </code></pre>
  
  <p>ghc looks like it's 64-bit:</p>
  
  <pre><code>$ ghc --info
   [("Project name","The Glorious Glasgow Haskell Compilation System")
   ,("GCC extra via C opts"," -fwrapv")
   ,("C compiler command","/usr/bin/llvm-gcc")
   ,("C compiler flags"," -m64 -fno-stack-protector  -m64")
   ,("ar command","/usr/bin/ar")
   ,("ar flags","clqs")
   ,("ar supports at file","NO")
   ,("touch command","touch")
   ,("dllwrap command","/bin/false")
   ,("windres command","/bin/false")
   ,("perl command","/usr/bin/perl")
   ,("target os","OSDarwin")
   ,("target arch","ArchX86_64")
   ,("target word size","8")
   ,("target has GNU nonexec stack","False")
   ,("target has subsections via symbols","True")
   ,("Project version","7.4.2")
   ,("Booter version","7.4.2")
   ,("Stage","2")
   ,("Build platform","x86_64-apple-darwin")
   ,("Host platform","x86_64-apple-darwin")
   ,("Target platform","x86_64-apple-darwin")
   ,("Have interpreter","YES")
   ,("Object splitting supported","NO")
   ,("Have native code generator","YES")
   ,("Support SMP","YES")
   ,("Unregisterised","NO")
   ,("Tables next to code","YES")
   ,("RTS ways","l debug  thr thr_debug thr_l thr_p  dyn debug_dyn thr_dyn thr_debug_dyn")
   ,("Leading underscore","YES")
   ,("Debug on","False")
   ,("LibDir","/usr/local/Cellar/ghc/7.4.2/lib/ghc-7.4.2")
   ,("Global Package DB","/usr/local/Cellar/ghc/7.4.2/lib/ghc-7.4.2/package.conf.d")
   ,("Gcc Linker flags","[\"-m64\"]")
   ,("Ld Linker flags","[\"-arch\",\"x86_64\"]")
   ]
  </code></pre>