Documentation/Makefile: allow man.base.url.for.relative.link to be set from Make

Signed-off-by: Junio C Hamano <junio@kernel.org>

Documentation/Makefile

@@ -103,6 +103,10 @@ ifdef DOCBOOK_SUPPRESS_SP
 XMLTO_EXTRA += -m manpage-suppress-sp.xsl
 endif
 
+ifdef MAN_BASE_URL
+XMLTO_EXTRA += --stringparam man.base.url.for.relative.links=$(MAN_BASE_URL)
+endif
+
 # If your target system uses GNU groff, it may try to render
 # apostrophes as a "pretty" apostrophe using unicode.  This breaks
 # cut&paste, so you should set GNU_ROFF to force them to be ASCII