Fix compilation on OSX 10.9

The new CLang compiler which ships with XCode 5.1 throws errors for unknown
compiler flags. This patch ignores those errors until the issue is fixed
upstream.

Basically, detects if the platform is darwin (OSX) and appends a new flag to
'cflags'.
Merge pull request #1 from joaoventura/clang

Fix compilation on OSX 10.9