emscripten: rely on emcc's EMSCRIPTEN define

Instead of having our own USING_EMSCRIPTEN define, we now just rely on
emcc's EMSCRIPTEN define.

cogl/cogl-renderer.c

@@ -151,7 +151,7 @@ static CoglDriverDescription _cogl_drivers[] =
     COGL_GLES2_LIBNAME,
   },
 #endif
-#ifdef USING_EMSCRIPTEN
+#ifdef EMSCRIPTEN
   {
     COGL_DRIVER_WEBGL,
     "webgl",

cogl/winsys/cogl-winsys-sdl.c

@@ -95,7 +95,7 @@ static CoglBool
 _cogl_winsys_renderer_connect (CoglRenderer *renderer,
                                CoglError **error)
 {
-#ifdef USING_EMSCRIPTEN
+#ifdef EMSCRIPTEN
   if (renderer->driver != COGL_DRIVER_GLES2)
     {
       _cogl_set_error (error, COGL_WINSYS_ERROR,
@@ -190,7 +190,7 @@ _cogl_winsys_display_setup (CoglDisplay *display,
       SDL_GL_SetAttribute (SDL_GL_CONTEXT_MINOR_VERSION, 0);
       break;
 
-#elif defined (USING_EMSCRIPTEN)
+#elif defined (EMSCRIPTEN)
     case COGL_DRIVER_GLES2:
       sdl_display->video_mode_flags = SDL_OPENGL;
       break;

configure.ac

@@ -304,7 +304,6 @@ AS_IF([test "x$enable_emscripten" = "xyes"],
         enable_sdl=yes
         enable_gles2=yes
         enable_gl=no
-        AC_DEFINE([USING_EMSCRIPTEN], 1, [Cogl is being compiled with emscripten])
         COGL_DEFINES_SYMBOLS="$COGL_DEFINES_SYMBOLS COGL_HAS_EMSCRIPTEN_SUPPORT"
       ]
 )