lang/perl: Use target headers for external modules

Before, external modules actually used host-perl headers.
Aside from obviously being the wrong thing to do, it made them misbehave if
host and target architectures differ too much.

Signed-off-by: Marcel Denia <naoir@gmx.net>

lang/perl/perlmod.mk

@@ -63,6 +63,7 @@ define perlmod/Configure
 		LINKTYPE=dynamic \
 		DESTDIR=$(PKG_INSTALL_DIR) \
 	);
+	sed 's!^PERL_INC = .*!PERL_INC = $(STAGING_DIR)/usr/lib/perl5/5.20/CORE/!' -i $(PKG_BUILD_DIR)/Makefile
 endef
 
 define perlmod/Compile