BUILD: time: remove the test on _POSIX_C_SOURCE

It seems it's not defined on FreeBSD while it's mentioned on Linux that
clock_gettime() can be detected using this. Given that we also have the
test for _POSIX_TIMERS>0 that should cover it well enough. If it breaks
on other systems, we'll see.

Report was here :
    https://github.com/haproxy/haproxy/runs/133866993
2 files changed