CMake: Error out if trying to build generic builds on Windows

Generic builds have never been supported on Windows.

CMakeLists.txt

@@ -199,6 +199,9 @@ else()
 endif()
 
 if(ENABLE_GENERIC)
+  if(WIN32)
+    message(FATAL_ERROR "Generic builds are not supported on Windows!")
+  endif()
   message(STATUS "Warning! Building generic build!")
   set(_M_GENERIC 1)
   add_definitions(-D_M_GENERIC=1)