Makefile.dist: Added support for VC7

Currently VC7 and VC7.1 builds have to be ran with the VC variable set
to vc6 which is not only inconsistent with the nmake winbuild system
but also with newer versions of Visual Studio supported by this file.

Note: This doesn't break the build for anyone still running with the
VC variable set to vc6 or not set (which defaults to vc6).