Fix builds from menu on Windows
When compiling for Windows via MinGW using CMake the variable RADIANT_EXECUTABLE is incorrectly set to ".exe" (it should be just "exe") and that prevents the user to build from Radiant menu.
When compiling for Windows via MinGW using CMake the variable RADIANT_EXECUTABLE is incorrectly set to ".exe" (it should be just "exe") and that prevents the user to build from Radiant menu.