build: Fix clock_gettime check with XCode 8

build: Fix clock_gettime check with XCode 8

With XCode 8, clock_gettime will be incorrectly detected as being
available regardless of what OS X version we're targetting because the
symbol is available in the .tbd library as a weak symbol.
See: https://github.com/Homebrew/homebrew-core/issues/3727#issue-170086273

It's only starting from macOS 10.12 that clock_gettime is actually
available, so we can unconditionally disable it when targetting older
versions. We cannot simply do AC_CHECK_FUNCS with -Wl,-no_weak_imports
because the autoconf check does its own prototype declaration that
doesn't trigger that compiler flag.

https://bugzilla.gnome.org/show_bug.cgi?id=772451