Once we have a .vcproj, and if we can exec devenv.exe, then prefer it.

Once we have a .vcproj, and if we can exec devenv.exe, then prefer it.
If we don't, we still need to use the .mak files, when present, because
a VC5 user (yes it still works) can't build from .dsp files.

But let the user do whatever they like, as trivial as USEMAK=1
to override the presense of .vcproj files, which is a huge improvement
over the prior detection.

Backport: r569352

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@569354 13f79535-47bb-0310-9956-ffa450edef68