clock_gettime is not used on Android, even though available (1.6.0+)

[hungrycat]

Describe the bug
Android has its own implementation of the system libraries: https://android.googlesource.com/platform/bionic.
clock_gettime is made available in libc, not in librt like on some other POSIX platforms. Yet, the check in the build system assumes clock_gettime is in librt:

From src/platform/posix/CMakeLists.txt:

nng_check_lib(rt clock_gettime NNG_HAVE_CLOCK_GETTIME)

As a consequence, on Android NNG_HAVE_CLOCK_GETTIME is never set, and fallback to nni_clock is used instead of clock_gettime. As documented in the file, this severely hinders stability of nng on that platform

To Reproduce
Build for Android, assert in posix_clock.c on which implementation is used. Expectation: clock_gettime.

Environment Details

• NNG version: started in 1.6.0, with this change: Fallback not used when clock_gettime is unavailable (crash on OSX < 10.12) #1576
• Operating system and version: Android
• Compiler and language used: NDK 26.1.10909125
• Shared or static library: should not matter

[hungrycat]

Possible solution:
clock_gettime.patch

[gdamore]

A better solution IMO is to look first in libc, and only look in librt if we didn't find it in libc.