build, fix check for MAP_ANON_NEEDS_DARWIN_SOURCE (issue #11)

Without _POSIX_C_SOURCE we would find always MAP_ANON or MAP_ANONYMOUS.
But any Makefile.am has CPPFLAGS with -D_POSIX_C_SOURCE=200112L
and -D_BSD_SOURCE.
Merge remote-tracking branch 'rudi/fixes'

* rudi/fixes:
  build, fix check for MAP_ANON_NEEDS_DARWIN_SOURCE (issue #11)