- move actual variables into com_MSC_class.mk
- use export ... && to set the variables, so they affect all subsequent
  commands, not just the first one
- clear MAKE as well, as that is apparently used by nmake, but can
  only point to GNU make
- set CC, because nmake apparently can interpret C:/Progra~1/.../cl.exe
  etc. etc. as a "C:" command with some additional arguments which only
  changes the current directory, without even invoking the shell, which
  tends to cause profanities to be uttered for extended periods of time

Change-Id: Ia7b1e6a70d6ac116d4ef0312d2aa1a4747fb8cbf
Reviewed-on: https://gerrit.libreoffice.org/44159Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>