cut win64 build over

contrib/cross/mingw.cmake

@@ -1,15 +1,11 @@
 set(CMAKE_SYSTEM_NAME Windows)
 set(TOOLCHAIN_PREFIX x86_64-w64-mingw32)
 
-if($ENV{ALT} MATCHES "1")
-set(TOOLCHAIN_PREFIX amd64-w64-mingw32)
-endif()
-
 add_definitions("-DWINNT_CROSS_COMPILE")
 
 # target environment on the build host system
 # second one is for non-root installs
-set(CMAKE_FIND_ROOT_PATH /usr/${TOOLCHAIN_PREFIX} /home/$ENV{USER}/mingw32 /home/$ENV{USER}/mingw64 /home/$ENV{USER}/mingw64/${TOOLCHAIN_PREFIX} /home/$ENV{USER}/mingw32/${TOOLCHAIN_PREFIX})
+set(CMAKE_FIND_ROOT_PATH /usr/${TOOLCHAIN_PREFIX} /opt/mingw64 /home/$ENV{USER}/mingw32 /home/$ENV{USER}/mingw64 /home/$ENV{USER}/mingw64/${TOOLCHAIN_PREFIX} /home/$ENV{USER}/mingw32/${TOOLCHAIN_PREFIX})
 
 # modify default behavior of FIND_XXX() commands
 set(CMAKE_FIND_ROOT_PATH_MODE_PROGRAM NEVER)