Patch `configure` and the shipped `intltool.m4` macro file to make a missing
XML::Parser perl module nonfatal.

All utilities provided by intltool are programmed to fail gracefully at run
time if the module cannot be loaded and those utilities commonly used during
the build do not require it at all.

Also drop the the build-prereq on XML::Parser now that it is optional.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>