Android build fixes

-Enabling checking for GCC_ATOMICS also on clang by default. This way all Android ABIs build successfully
 -Android cmake: Threading was not enabled correctly
 -Android cmake: Timers and dynamic lib loading were not included in the sources

cmake/sdlchecks.cmake

@@ -859,6 +859,8 @@ macro(CheckPTHREAD)
     if(LINUX)
       set(PTHREAD_CFLAGS "-D_REENTRANT")
       set(PTHREAD_LDFLAGS "-pthread")
+    elseif(ANDROID)
+      # pthreads are builtin
     elseif(BSDI)
       set(PTHREAD_CFLAGS "-D_REENTRANT -D_THREAD_SAFE")
       set(PTHREAD_LDFLAGS "")