Fix perl/ build.

An earlier commit f848718a broke the build in perl/ directory by
allowing the Makefile.PL to overwrite the now-tracked Makefile.
Fix this by forcing Makefile.PL to produce its output in
perl.mak as the broken commit originally intended.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

perl/Makefile

@@ -29,7 +29,7 @@ $(makfile): ../GIT-CFLAGS Makefile
 	echo '	echo $(instdir_SQ)' >> $@
 else
 $(makfile): Makefile.PL ../GIT-CFLAGS
-	'$(PERL_PATH_SQ)' $< FIRST_MAKEFILE='$@' PREFIX='$(prefix_SQ)'
+	'$(PERL_PATH_SQ)' $< PREFIX='$(prefix_SQ)'
 endif
 
 # this is just added comfort for calling make directly in perl dir

perl/Makefile.PL

@@ -24,5 +24,6 @@ WriteMakefile(
 	NAME            => 'Git',
 	VERSION_FROM    => 'Git.pm',
 	PM		=> \%pm,
+	MAKEFILE	=> 'perl.mak',
 	%extra
 );