x64: add toolchain file for x64 Windows compilation

Change-Id: I319f15b3227b1ffd6cc5dbd3ab65515d167ac387
Tuleap: #387

cmake/winBuild64.cmake

+# this one is important
+SET(CMAKE_SYSTEM_NAME Windows)
+
+# specify the cross compiler
+SET(CMAKE_C_COMPILER   x86_64-w64-mingw32-gcc)
+SET(CMAKE_CXX_COMPILER x86_64-w64-mingw32-g++)
+SET(CMAKE_RC_COMPILER x86_64-w64-mingw32-windres)
+SET(CMAKE_ASM_YASM_COMPILER yasm)
+SET(CMAKE_FIND_ROOT_PATH /usr/x86_64-w64-mingw32/)
+set(LIB_FLAGS "-Wl,--output-def,libringclient.def")