Fixed MinGW detection.

CMakeLists.txt

@@ -175,7 +175,7 @@ if (_GLFW_WIN32)
     # the inclusion of stddef.h (by glfw3.h), which is itself included before
     # win32_platform.h.  We define them here until a saner solution can be found
     # NOTE: MinGW-w64 and Visual C++ do /not/ need this hack.
-    if (CMAKE_COMPILER_IS_GNUC)
+    if (${CMAKE_C_COMPILER_ID} STREQUAL "GNU")
         add_definitions(-DUNICODE -DWINVER=0x0501)
     endif()
 endif()