perl: fix installing modules from contrib

Commit 20d2a30f (Makefile: replace perl/Makefile.PL with simple make rules)
removed a target that allowed Makefiles from contrib/ to get the correct
install path. This introduces a new target for main Makefile and fixes
installation for Mediawiki module.

v2: Pass prefix as that can have influence as well, add single quotes
    for _SQ variant.

Signed-off-by: Christian Hesse <mail@eworm.de>

Makefile

@@ -1973,6 +1973,8 @@ GIT-PERL-DEFINES: FORCE
 		echo "$$FLAGS" >$@; \
 	    fi
 
+perllibdir:
+	@echo '$(perllibdir_SQ)'
 
 .PHONY: gitweb
 gitweb:

contrib/mw-to-git/Makefile

@@ -21,8 +21,9 @@ HERE=contrib/mw-to-git/
 INSTALL = install
 
 SCRIPT_PERL_FULL=$(patsubst %,$(HERE)/%,$(SCRIPT_PERL))
-INSTLIBDIR=$(shell $(MAKE) -C $(GIT_ROOT_DIR)/perl \
-                -s --no-print-directory instlibdir)
+INSTLIBDIR=$(shell $(MAKE) -C $(GIT_ROOT_DIR)/ \
+                -s --no-print-directory prefix=$(prefix) \
+                perllibdir=$(perllibdir) perllibdir)
 DESTDIR_SQ = $(subst ','\'',$(DESTDIR))
 INSTLIBDIR_SQ = $(subst ','\'',$(INSTLIBDIR))