Change compilation of memcached module to produce PIE (position-independent
executable) instead of a regular shared object.

The nice thing about the PIE executable is that it can also run on Linux,
while ".so" can't run on Linux without tricks (easy tricks, granted, but
it's still not natural to run a ".so" on Linux).

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>