gitweb: fallback to system-wide config file (fixup)

The earlier one did not correctly propagate GITWEB_CONFIG_SYSTEM from
Makefile to generated gitweb.cgi script.

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -1035,6 +1035,7 @@ gitweb/gitweb.cgi: gitweb/gitweb.perl
 	    -e 's|++GIT_VERSION++|$(GIT_VERSION)|g' \
 	    -e 's|++GIT_BINDIR++|$(bindir)|g' \
 	    -e 's|++GITWEB_CONFIG++|$(GITWEB_CONFIG)|g' \
+	    -e 's|++GITWEB_CONFIG_SYSTEM++|$(GITWEB_CONFIG_SYSTEM)|g' \
 	    -e 's|++GITWEB_HOME_LINK_STR++|$(GITWEB_HOME_LINK_STR)|g' \
 	    -e 's|++GITWEB_SITENAME++|$(GITWEB_SITENAME)|g' \
 	    -e 's|++GITWEB_PROJECTROOT++|$(GITWEB_PROJECTROOT)|g' \