Fix 'make dist' broken since 35287878

In commit 35287878b9785efeaeb854afb6ee4509f5714d65:

    [xbuild] Reorganize xbuild targets/frameworks/bin data

Several files were shuffled around, but the EXTRA_DISTFILES was
not fully updated with the new file paths.

This patch is required to build a 3.2.7 tarball.
[xbuild] Define VisualStudioVersion, since F# projects can depend on it
[xbuild] Fix PCL 4.5 targets