doc: Don't fail if manpages fail to build

Even with installed asciidoc and xsltproc the build will fail if
xsltproc cannot find the required stylesheet
http://docbook.sourceforge.net/release/xsl/current/manpages/docbook.xsl.

Ignore such errors until there is a better check in configure.

Signed-off-by: Stefan Weil <[email protected]>

Author: stweil

doc/Makefile.am

@@ -37,7 +37,7 @@ html: ${man_MANS:%=%.html}
 SUFFIXES = .asc .html
 
 .asc:
-	asciidoc -b docbook -d manpage -o - $< | \
+	-asciidoc -b docbook -d manpage -o - $< | \
 	xsltproc --nonet $(man_xslt) -
 
 .asc.html: