docs: make 'gmake dist-docs' work

doc: buildsystem fixes

- use correct pathes (make gmake dist-docs from toplevel directory work)
- configure detects perl as tools/configure does

Signed-off-by: Christoph Egger <chegger@amazon.de>
Reviewed-by: Matthew Wilson <msw@amazon.de>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- reran autogen.sh ]

config/Docs.mk.in

@@ -11,3 +11,4 @@ POD2MAN             := @POD2MAN@
 POD2HTML            := @POD2HTML@
 POD2TEXT            := @POD2TEXT@
 MARKDOWN            := @MARKDOWN@
+PERL                := @PERL@