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.
Include xbuild_4_0_dirs in dist; fixes 425fdc03

In commit 425fdc03fa38b2ee29c1cf88617ddb1580fde78f:

  [xbuild] Build & install MSBuild 12.0 assemblies

The net_3_5_only_dirs containing the Build classes were moved to
various xbuild_X_dirs variables, but an undefined $(xbuild_dirs)
variable was appended to DIST_ONLY_SUBDIRS.

This patch adds $(xbuild_4_0_dirs) instead.