man: fix manpage build instructions

This fixes all the out-of-tree build-failures with manpages and uses a
.man_fixup file to avoid overriding man-pages on every build.

Manpages are only built if xsltproc is found and the stylesheets are
available locally. You can disable building manpages with
--disable-manpages so the quite expensive xsltproc procedure can be
skipped.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>