configure.ac: Consider Mac OS X target version for clock_gettime()

configure.ac: Consider Mac OS X target version for clock_gettime()

The AC_CHECK_FUNCS() macro checks whether a program compiled on the host
will link against the function, but it does not help when targeting
older systems where that function is not available. Additionally, the
way that autoconf compiles the program ignores any attributes or
preprocessor guards that installed headers might have applied to that
function.

Resolve this by verifying that the OS X target version is 10.12 or newer
before checking for clock_gettime().

Closes #836

Signed-off-by: Chris Dickens <christopher.a.dickens@gmail.com>