remove built man page on make clean.
(maybe this should be for maintainer clean only?)

Use $XSLTPROC macro instead of xsltproc.

Generate dummy manpage if --enable-man is not defined.
This idea came from pango source tree.
This is workaround so that the dist tarball includes the manpage.
(Some is commented out; this is not complete yet.)

This is because "make dist" needs the manpages but I don't want normal
"make" builds to require building them due to more build dependencies.