Added -D_DARWIN_C_SOURCE=1 to compiler flags to ensure non-default exten...

I found this solution which eliminates the build warning when built on OSX Clang stating that snprintf is not defined.

By default, when POSIX compliance is enabled, snprintf, and some other extensions are not available in the default stdio inclusion. Adding this flag re-enables these extensions, in addition to the POSIX requirements.

I stumbled on this patch in [this thread](http://sourceforge.net/p/gphoto/bugs/935/)