Fixed compiling test program with Emscripten.

test/testgles2.c

@@ -20,7 +20,7 @@
 
 #include "SDL_test_common.h"
 
-#if defined(__IPHONEOS__) || defined(__ANDROID__) || defined(__NACL__)
+#if defined(__IPHONEOS__) || defined(__ANDROID__) || defined(__EMSCRIPTEN__) || defined(__NACL__)
 #define HAVE_OPENGLES2
 #endif