Install man3 manpages to $PREFIX/share/man/man3 even for site installs

MakeMaker supports three installation modes: perl, site, and vendor. The first
and third install manpages to $PREFIX/share/man, only site installs to
$PREFIX/man. For consistency with the rest of git, which does not make the
distinction and writes all manpages to $PREFIX/share/man, this change makes
sure that perl does too, even when it's installed in site mode.

Signed-off-by: martin f. krafft <madduck@madduck.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

perl/Makefile.PL

@@ -29,5 +29,6 @@ WriteMakefile(
 	VERSION_FROM    => 'Git.pm',
 	PM		=> \%pm,
 	MAKEFILE	=> 'perl.mak',
+	INSTALLSITEMAN3DIR => '$(SITEPREFIX)/share/man/man3',
 	%extra
 );