generated man pages: build without hard coded extension

generated man pages: build without hard coded extension

The automake MAN primary requires a hard coded extension to build
man pages. Let's avoid that as the extension number may vary by platform.
Take advantage of the fact that the man directory only contains man pages.
Wildcards are not supported by Automake but it happens to work
sufficiently well here.

Normally xorg build man pages by converting a source .man file to a
target file with the extension number. That would be too many files
in this case.

Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Gaetan Nadon <memsize@videotron.ca>