Pass .def files directly to MinGW tools (#9997)

Commit 6a61a8a5 (Honor module .def files with MinGW tools, 2011-02-21)
set CMAKE_LINK_DEF_FILE_FLAG to "-Wl," to enable passing .def files to
the linker on MinGW.  However, older GNU tools in the MSYS shell do not
know how to translate "-Wl,/c/..." to "c:/..." and complain that the
file does not exist.  Instead set the flag to just "" which tells CMake
it can pass the file through the front-end with no special flag.

Modules/Platform/Windows-GNU.cmake

@@ -43,7 +43,7 @@ endif()
 set(CMAKE_DL_LIBS "")
 set(CMAKE_LIBRARY_PATH_FLAG "-L")
 set(CMAKE_LINK_LIBRARY_FLAG "-l")
-set(CMAKE_LINK_DEF_FILE_FLAG "-Wl,")
+set(CMAKE_LINK_DEF_FILE_FLAG "") # Empty string: passing the file is enough
 set(CMAKE_LINK_LIBRARY_SUFFIX "")
 set(CMAKE_CREATE_WIN32_EXE  "-mwindows")