libfaketime.c: wrap timespec_get in TIME_UTC macro

Function `timespec_get` is not guaranteed to be declared in MacOS
since its standard library is non-conformance to modern standards.
Therefore, skip patching `timespec_get` if it is undeclared by the
standard library.

The detection of `timespec_get` is based on the conjecture that the
macro `TIME_UTC` is only defined when `timespec_get` is declared.