Always forcing `--mandir=' in WAF_ARGS can be problematic for waf-s which does
not support that (e.g. nanomsgxx, ardour). Pointed out by <kamil>.

Index: waf.mk
===================================================================
RCS file: /cvsroot/pkgsrc/devel/waf/waf.mk,v
retrieving revision 1.2
diff -u -p -r1.2 waf.mk
--- waf.mk	24 May 2015 14:06:42 -0000	1.2
+++ waf.mk	18 Jun 2016 09:10:19 -0000
@@ -18,7 +18,10 @@ WAF_ENV+=	JOBS=${MAKE_JOBS:Q}
 WAF_ENV+=	LC_ALL="en_US.UTF-8"
 .endif
 
-WAF_ARGS=	--mandir=${PREFIX}/${PKGMANDIR}
+WAF_HAS_MANDIR?=	yes
+.if !empty(WAF_HAS_MANDIR:M[yY][eE][sS])
+WAF_ARGS=		--mandir=${PREFIX}/${PKGMANDIR}
+.endif
 
 post-extract:
 	cp ${PREFIX}/bin/waf ${WRKSRC}