Fix "FindGLFW3.cmake" to work with MinGW.
Fix "FindGLFW3.cmake" to work with MinGW.

--HG--
branch : mingw

cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two,cbeck88/cegui-mirror-two