This requires a newish gcc to build. gcc 4.7 will work.
We do not require gcc as a build dependency because we want to make sure that conda build finds all the files installed for this gcc when creating the package. So you will need to make sure that the gcc is on the PATH independently.
On OS X, the dylibs libgcc_ext.10.4.dylib and libgcc_ext.10.5.dylib are Mach-O stub files, which install_name_tool cannot modify. So we have to use the binary replacement to change the RPATH in those files, and in a few other dylibs that pull in the paths from those files.