compat: enable building on OS X

OS X lacks some Linux (and recent POSIX) specific functionality; implement
the required OS X equivalents in compat.h and add OS X Makefile rules in
compat.mk.

Switched tests to use CLOCK_REALTIME as they don't really need monotonic
clock for anything and it makes compatibility easier.

In the Python CFFI wrapper, load the .dylib module on darwin.
test_commands: don't test touch and gat with memcached < 1.4.8

Touch and GAT were new features in 1.4.8, doesn't make sense to try to test
them if we're running something older.
test_omcache: exit early if memcached doesn't exist

Previously we tried to run the tests even if we couldn't execute memcached
which made little sense as all the tests would fail with silly errors.