Change occurrences of gcc to the more general cc

This is done for the sake of Yosemite, which does not have gcc, and yet
this change is also compatible with Linux.

pkgs/development/libraries/botan/generic.nix

@@ -17,9 +17,11 @@ stdenv.mkDerivation rec {
   buildInputs = [ python bzip2 zlib gmp openssl boost ];
 
   configurePhase = ''
-    python configure.py --prefix=$out --with-gnump --with-bzip2 --with-zlib --with-openssl
+    python configure.py --prefix=$out --with-gnump --with-bzip2 --with-zlib --with-openssl --cc=$CC
   '';
 
+  enableParallelBuilding = true;
+
   postInstall = ''
     cd "$out"/lib/pkgconfig
     ln -s botan-*.pc botan.pc || true