Conditionally add -Wno-unused-local-typedefs

This option is not supported on mac compiler
i686-apple-darwin11-llvm-gcc-4.2 (GCC) 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2336.1.00)

Closes #258
Merge pull request #259 from finetjul/258-fix-no-unused-local-typedefs-on-mac

Conditionally add -Wno-unused-local-typedefs