add support for building R14 on Mountain Lion and newer

Xcode compiler has changed to clang in OS X 10.8 and 10.9. This prevents Erlang R14 from being built from source due to a bug in the way the __inline__ operator was assumed to work in the Erlang code. Both R15 and R16 have a slightly different code structure for inline functions that work with these newer compilers. 

This patch will determine the Darwin OS version [11 = 10.7 (Lion), 12 = 10.8 (Mountain Lion) 13 = 10.9 (Mavericks)] and apply a code patch to R14 and pass in a proper C definition for a successful build.

Tested using R14B04, OS X 10.7, OS X 10.8 and OS X 10.9.