The VS >= 10 generators need to parse the .sln file from the build
directory to locate targets in subdirectories. This occurs after we
change the working directory to the build tree. If a relative directory
other than "." was given then we would change to it and also refer to
the .sln file location with it. Fix this by converting the build tree
to a full path always. This will also give a more informative error
message when the directory does not exist.
Brad King