Move ugly _POSIX_SOURCE and _POSIX_C_SOURCE=200112L flags to pipe_posix.c.

They are only needed for popen and pclose, so let's define them just
in the one file that uses popen and pclose. This way it's obvious why
we need to define them (because stupid GNU requires them for the
*standardized* popen and pclose calls).
Merge pull request #88 from eaburns/dev/eaburns

Move ugly _POSIX_SOURCE and _POSIX_C_SOURCE=200112L flags to pipe_posix.c