I thought it was possible now to build on Windows with Visual Studio
2013 as the only installed Visual Studio version, but no. I tried on a
fresh Windows 8.1 installation.

This commit fixes the configury a bit at least. (One needs to pass the
--with-visual-studio=2013 option. Otherwise configure gets confused by
the partial (?) VS2012 that seems to be installed, too, when
installing VS2013, and prefers that...)

The build fails at least in external/lcms2, but I'll leave sorting out
that for later.

Change-Id: I15942e4b088a3f0a62c3f7fa8f9b45f77beaff6f