Support POSIX, GNU and OS specific flavours of standard headers

[jonas]

Often C standard headers allow to specify which specification to conform to via defines. For example _POSIX_C_SOURCE. With the these defines can be passed with -extra-arg:

./scalaBindgen /usr/include/ctype.h -name ctype \
    -extra-arg=-D_POSIX_C_SOURCE \
    -extra-arg=-D_DONT_USE_CTYPE_INLINE_

The goal of this ticket is to think about how could provide a way to target a specific specification across different platforms.

[ekrich]

I think this a very good idea - typical headers on macOS have lots of definitions not related to Std C or POSIX that seem to create lots of noise.