Documentation/Makefile: make AsciiDoc dblatex dir configurable

On my system this is in /usr/share/asciidoc/dblatex not
/etc/asciidoc/dblatex.  Extract this portion of the path to a variable
so that is can be set in config.mak.

Signed-off-by: John Keeping <john@keeping.me.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>

Documentation/Makefile

@@ -103,6 +103,7 @@ MAKEINFO = makeinfo
 INSTALL_INFO = install-info
 DOCBOOK2X_TEXI = docbook2x-texi
 DBLATEX = dblatex
+ASCIIDOC_DBLATEX_DIR = /etc/asciidoc/dblatex
 ifndef PERL_PATH
 	PERL_PATH = /usr/bin/perl
 endif
@@ -354,7 +355,7 @@ user-manual.texi: user-manual.xml
 
 user-manual.pdf: user-manual.xml
 	$(QUIET_DBLATEX)$(RM) $@+ $@ && \
-	$(DBLATEX) -o $@+ -p /etc/asciidoc/dblatex/asciidoc-dblatex.xsl -s /etc/asciidoc/dblatex/asciidoc-dblatex.sty $< && \
+	$(DBLATEX) -o $@+ -p $(ASCIIDOC_DBLATEX_DIR)/asciidoc-dblatex.xsl -s $(ASCIIDOC_DBLATEX_DIR)/asciidoc-dblatex.sty $< && \
 	mv $@+ $@
 
 gitman.texi: $(MAN_XML) cat-texi.perl