cmake: do not bind to any particular POSIX C standard

cmake: do not bind to any particular POSIX C standard

This caused build failures on FreeBSD. Defining _POSIX_C_SOURCE to
a particular version will disable common non-POSIX extensions like
PF_UNIX, and on some systems will also disable features of later
POSIX versions, like IPv6. If we don't ask for a specific version,
we'll get some sort of sensible default.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=66257
Signed-off-by: Chengwei Yang <chengwei.yang@intel.com>
[made the commit message more concise -smcv]
Reviewed-by: Simon McVittie <simon.mcvittie@collabora.co.uk>