Blender Git Loki

Windows: Improve rebuild.cmd

Previously the MSVC environment was setup every time using vcvarsall.bat
when you ran you ran rebuild.cmd, Generally not an issue but after many
rebuilds on the same console, it grows the path environment variable
beyond what is supported and building breaks.

This patch adds a check to see if the environment is setup already
and skips the call to vcvarsall.bat

Also cleans up the double build in the msbuild's version of rebuild.cmd
install.vcxproj will build all that is needed, so no need to do a regular
build first.