configure: add generic pshared mutex test

configure: add generic pshared mutex test

Add a feature test to configure to determine if a platform has
interprocess synchronization primitives that work with threads and
remove the hardcoded enablement in the individual os/os-* headers. This
should work better for platforms where support was overlooked or didn't
exist at some point in the past but has since been added. Also update
the README to not single out FreeBSD for being thread only as this might
change...

This is based off work by the Boost project
(https://github.com/boostorg/interprocess/blob/boost-1.63.0/include/boost/interprocess/detail/workaround.hpp#L45
) that found that OSX 10.6 and Cygwin are buggy even though
_POSIX_THREAD_PROCESS_SHARED says this feature should be work on them.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>