Decouple mod-test.c from Gnulib

Decouple mod-test.c from Gnulib

mod-test.c should not use Gnulib code, as that creates
unnecessary coupling between Emacs internals and this test module.
Also Gnulib code is compiled without -fPIC whereas mod-test.c
needs -fPIC and recompiling Gnulib with -fPIC would be too painful.
* src/Makefile.in (LIB_NANOSLEEP): New macro.
(LIBES): Use it.
* test/Makefile.in (REPLACE_FREE, FREE_SOURCE_0, FREE_SOURCE_0):
Remove.  All uses removed.
(LIB_CLOCK_GETTIME, LIB_NANOSLEEP): New macros.
(MODULE_CFLAGS): Do not -I from lib as that would include Gnulib
modifications to standard .h files (e.g., "#define nanosleep
rpl_nanosleep") and we don’t want the Gnulib replacements.
Instead, for gmp.h (on platforms lacking <gmp.h>) simply '-I.'
with a suitable gmp.h.
(gmp.h): New rule to create a suitable gmp.h.
($(test_module)): Depend on config.h since the code uses config.h.
Depend on gmp.h if we need to create a suitable one.
If compiling mini-gmp.h, compile the original one and not
the Emacs-specific one, to lessen coupling with Emacs internals.
Link with LIB_CLOCK_GETTIME and LIB_NANOSLEEP.
(clean): Remove gmp.h.
* test/src/emacs-module-resources/mod-test.c: Don’t include timespec.h.
All uses of timespec.h APIs changed to use system-supplied APIs.
Change _Static_assert to plain assert, so that we needn’t rely
on Gnulib’s _Static_assert.
(timespec_le) [CLOCK_REALTIME]: New function.
Change use of timespec_cmp changed to use this instead.
(Fmod_test_sleep_until, Fmod_test_nanoseconds):
Define these functions and their Lisp names mod-test-sleep-until
and mod-test-nanoseconds only if CLOCK_REALTIME,
since they now won’t work on platforms lacking CLOCK_REALTIME.
(Fmod_test_nanoseconds): Just use _Static_assert since it
should work on all platforms.
* test/src/emacs-module-tests.el (mod-test-sleep-until)
(mod-test-nanoseconds, mod-test-double):
Skip test if the corresponding functione is not defined.