cmake: Fix UI being dependent on browser plugin files

The UI currently is dependent on browser source files being present,
when in actuality the files may not have been fetched (i.e. not fetching
submodules).  This fixes that by using a macro to detect whether it's
available or not.

Closes obsproject/obs-studio#1490

CMakeLists.txt

@@ -142,8 +142,8 @@ if(NOT INSTALLER_RUN)
 
 	add_subdirectory(libobs-opengl)
 	add_subdirectory(libobs)
-	add_subdirectory(UI)
 	add_subdirectory(plugins)
+	add_subdirectory(UI)
 	if (BUILD_TESTS)
 		add_subdirectory(test)
 	endif()