gettext: make build system look more like automake gettext

gettext: make build system look more like automake gettext

configure.in: Find all *.c files using find so no need to update
po/Makefile manually (previous method leads to problems like when
someone adds new .c file and forgets to update Makefile - then
translations are removed after update).

po/Makefile: Use automaticly found .c files. Add update-po target.
Show translation statistics when processing po files. Turn on msgfmt
validation (-c).

How to update translations now?

 $ ./configure
 $ cd po; make update-po
 $ vim your.po
 $ make

Signed-off-by: Arkadiusz Miśkiewicz <arekm@maven.pl>
Reviewed-by: Christoph Hellwig <hch@lst.de>