Parallel build with MSVC

On my system, "build DEBUG" takes ~2.75 minutes. When I tell MSBuild to
build in parallel by passing it the /m flag, that goes down to 1.5 minutes.

The first patch passes the value of the MSBFLAGS environment variable to
msbuild's command line.

The output of parallel and sequential builds has identical file count
and size after installing; all tests pass.

Even without /m, MSBuild will already run enough compilers to keep all
CPUs busy whenever it can do that with only a single project's files.
With /m, it will also process _projects_ in parallel.

One additional fix required is to put gendef.pl's "symbols.out" file
into the directory it is working on, rather than the source tree root,
to avoid collisions that cause the parallel build to fail otherwise.

--
Christian