better check for clock_gettime_nsec_np() -- cf. bug #5467.

CMakeLists.txt

@@ -873,7 +873,18 @@ if(LIBC)
     else()
       set(HAVE_ALLOCA_H 1)
       set(HAVE_ALLOCA 1)
-      check_symbol_exists(clock_gettime_nsec_np "time.h" HAVE_CLOCK_GETTIME_NSEC_NP)
+      if(IOS OR TVOS)   # assuming iOS / TVOS >= 10.0
+        set(HAVE_CLOCK_GETTIME_NSEC_NP 1)
+      else()
+        CHECK_C_SOURCE_COMPILES("
+          #include <AvailabilityMacros.h>
+          #include <time.h>
+          #if MAC_OS_X_VERSION_MIN_REQUIRED < 101200
+          #error Need 10.12 or newer target
+          #endif
+          int main(int argc, char **argv) { return !clock_gettime_nsec_np(CLOCK_MONOTONIC); }
+        " HAVE_CLOCK_GETTIME_NSEC_NP)
+      endif()
     endif()
 
     check_struct_has_member("struct sigaction" "sa_sigaction" "signal.h" HAVE_SA_SIGACTION)