auto remove empty /usr/share dir

Often times, the only reason /usr/share exists is because of documentation
installed into it.  So once we're done cleaning the docs, attempt to rmdir
/usr/share.  If it's empty, things will work out nicely.

[Peter: use make infrastructure to ignore errors rather than shell]
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>