fixes #1866 clock_gettime is not used on Android, even though available

Author: gdamore

src/platform/posix/CMakeLists.txt

@@ -29,7 +29,12 @@ if (NNG_PLATFORM_POSIX)
     nng_check_func(getrandom NNG_HAVE_GETRANDOM)
     nng_check_func(arc4random_buf NNG_HAVE_ARC4RANDOM)
 
-    nng_check_lib(rt clock_gettime NNG_HAVE_CLOCK_GETTIME)
+    nng_check_func(clock_gettime NNG_HAVE_CLOCK_GETTIME_LIBC)
+    if (NNG_HAVE_CLOCK_GETTIME_LIBC)
+        nng_defines_if(NNG_HAVE_CLOCK_GETTIME_LIBC NNG_HAVE_CLOCK_GETTIME)
+    else()
+        nng_check_lib(rt clock_gettime NNG_HAVE_CLOCK_GETTIME)
+    endif()
     nng_check_lib(pthread sem_wait NNG_HAVE_SEMAPHORE_PTHREAD)
     nng_check_lib(pthread pthread_atfork NNG_HAVE_PTHREAD_ATFORK_PTHREAD)
     nng_check_lib(pthread pthread_set_name_np NNG_HAVE_PTHREAD_SET_NAME_NP)