visual studio 2013 express

[Michael Stahl]

anybody has $SUBJECT installed?  /me doesn't, hence this mail...

on master configure fails with Express since commit
98a61f5b8da500fcefd2e7f0dfb1231bfb87f2fb  due to a check for devenv.exe,
which does not exist in  Express but there is a different executable,
probably vcexpress.exe.

does that executable support an /Upgrade switch to upgrade project and
solution files?  if not we'll have to drop support for VS Express at
some point and require VS Community.