git-remote-mediawiki: honor DESTDIR in "make install"

git-remote-mediawiki: honor DESTDIR in "make install"

So now you can run

	DESTDIR=$(pwd)/tmp make -Ccontrib/mw-to-git install

to install the mediawiki remote helper, git-mw tool, and Git::Mediawiki
perl module under tmp/ as preparation for zipping it up and extracting
on another machine.

While at it, make sure the directory that should contain Git::Mediawiki
exists before putting a file there.  Without this patch, the makefile
uses DESTDIR when installing git-mw and git-remote-mediawiki but not
the perl module, resulting in errors from "make install" if the
$(INSTLIBDIR)/Git directory does not exist:

 install: cannot create regular file \
 '/usr/share/perl/5.18.1/Git/Mediawiki.pm': No such file or directory

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Acked-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>