cmake: make sure check_* uses 2 different variables for clock_gettime

CMakeLists.txt

@@ -1536,13 +1536,13 @@ 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)
-    if(FOUND_CLOCK_GETTIME)
+    check_library_exists(rt clock_gettime "" FOUND_CLOCK_GETTIME_LIBRT)
+    if(FOUND_CLOCK_GETTIME_LIBRT)
       list(APPEND EXTRA_LIBS rt)
       set(HAVE_CLOCK_GETTIME 1)
     else()
-      check_library_exists(c clock_gettime "" FOUND_CLOCK_GETTIME)
-      if(FOUND_CLOCK_GETTIME)
+      check_library_exists(c clock_gettime "" FOUND_CLOCK_GETTIME_LIBC)
+      if(FOUND_CLOCK_GETTIME_LIBC)
         set(HAVE_CLOCK_GETTIME 1)
       endif()
     endif()