look for clock_gettime() in libc first, then in librt if not found.

CMakeLists.txt

@@ -1612,14 +1612,14 @@ elseif(UNIX AND NOT APPLE AND NOT RISCOS AND NOT HAIKU)
   CheckPTHREAD()
 
   if(SDL_CLOCK_GETTIME)
-    check_library_exists(rt clock_gettime "" FOUND_CLOCK_GETTIME_LIBRT)
-    if(FOUND_CLOCK_GETTIME_LIBRT)
-      list(APPEND SDL_EXTRA_LIBS rt)
+    check_library_exists(c clock_gettime "" FOUND_CLOCK_GETTIME_LIBC)
+    if(FOUND_CLOCK_GETTIME_LIBC)
       set(HAVE_CLOCK_GETTIME 1)
     else()
-      check_library_exists(c clock_gettime "" FOUND_CLOCK_GETTIME_LIBC)
-      if(FOUND_CLOCK_GETTIME_LIBC)
+      check_library_exists(rt clock_gettime "" FOUND_CLOCK_GETTIME_LIBRT)
+      if(FOUND_CLOCK_GETTIME_LIBRT)
         set(HAVE_CLOCK_GETTIME 1)
+        list(APPEND SDL_EXTRA_LIBS rt)
       endif()
     endif()
   endif()