buildenv/1.3.x/win32-static: fix libmcpp release build by completely clearing the DEBUG flag.
buildenv/1.3.x/win32-static: fix libmcpp release build by completely clearing the DEBUG flag.

The Makefile uses 'ifdef DEBUG', so setting DEBUG=0
still triggers a debug build.

Fix this by not passing the DEBUG flag at all for the
release build.

ccpgames/mumble-releng,ccpgames/mumble-releng