Fix from-scratch build breakage: Don't call realpath on Windows path

In a from-scratch build, when running the configure script, which then
runs the set_soenv script, the default_images symlink (that is passed
to this function in the form of a Windows path, for some reason) does
not exist yet, and realpath fails anyway. So don't bother calling
realpath on Windows paths.

Although I don't know whether it then will cause a problem that the
cygpath -m call won't be able to expand the symlink as it doesn't
exist anyway. This is a mess. And if cygpath -m expands symlinks
anyway, why is the realpath needed at all?

set_soenv.in

@@ -2338,7 +2338,7 @@ sub PathFormat
        # Replace absolute paths or DOS paths with ...
        if ( ( $variable =~ m/^\// ) or ( $variable =~ m/:/ ) ) {
            # mixed mode paths
-           chomp( $variable = qx{realpath "$variable"} );
+           chomp( $variable = qx{realpath "$variable"} ) unless ($variable =~ m/:/);
            chomp( $variable = qx{cygpath -m "$variable"} );
        }
    }