xmlto is a shell script, it needs to be fed MSYSsy filenames.
This patch adds a cygpath invocation for filename conversion (autotools do
that automatically, for CMake you have to spell it out). Cygwpath is available
in MSYS2 (and Cygwin, obviously).
When cygpath is not available, use MSYS-specific pwd extension to get W32 path.
Bug: https://bugs.freedesktop.org/show_bug.cgi?id=75860Reviewed-by: Ralf Habacker <email@example.com>