Fix parallel build in doc directory.

docbook2man has fixed output file names; place output in a subdirectory to
avoid collisions.
diff --git a/doc/Makefile.am b/doc/Makefile.am
index 7a9a7c2..b7c2812 100644
--- a/doc/Makefile.am
+++ b/doc/Makefile.am
@@ -118,9 +118,11 @@
 
 func.refs: local-fontconfig-devel.sgml $(DOC_FUNCS_SGML) version.sgml confdir.sgml
 	$(RM) func.refs
-	$(DOC2MAN) local-fontconfig-devel.sgml && \
-	mv manpage.refs func.refs
-	$(RM) manpage.links
+	$(DOC2MAN) -o devel-man local-fontconfig-devel.sgml && \
+	mv devel-man/manpage.refs func.refs && \
+	mv devel-man/*.3 . && \
+	$(RM) devel-man/manpage.links && \
+	rmdir devel-man
 
 local-fontconfig-devel.sgml: fontconfig-devel.sgml
 	$(LN_S) $< $@