support non-Linux systems with GNU userland
support non-Linux systems with GNU userland

Treat systems with GNU userland the same way as Linux.

This fixes build failures at least on:
- GNU/kFreeBSD (sys.platform == "gnukfreebsd10" or similar)
- GNU/Hurd (sys.platform == "gnu0")

sciyoshi/pyicu,sciyoshi/pyicu,sciyoshi/pyicu