Only give warning for 'bad' MinGW compiler when no SSE option given.

So that user does not receive the same warning subsequently after taking action to set the URHO3D_SSE option explicitly or after the correct setting has been set by the script.

Source/CMake/Modules/Urho3D-CMake-common.cmake

@@ -21,8 +21,8 @@
 #
 
 # Certain MinGW versions fail to compile SSE code. This is the initial guess for known "bad" version range, and can be tightened later
-if (MINGW)
-    execute_process(COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
+if (MINGW AND NOT DEFINED URHO3D_SSE)
+    execute_process (COMMAND ${CMAKE_C_COMPILER} -dumpversion OUTPUT_VARIABLE GCC_VERSION)
     if (GCC_VERSION VERSION_LESS 4.9.1)
         message (WARNING "Disabling SSE by default due to MinGW version. It is recommended to upgrade to MinGW with GCC >= 4.9.1. You can also try to re-enable SSE with CMake option -DURHO3D_SSE=1, but this may result in compile errors.")
         set (URHO3D_DEFAULT_SSE FALSE)