mandoc: Add -D_OPENBSD_SOURCE to CFLAGS on NetBSD

Uses both strtonum(3) and reallocarray(3).

Bump PKGREVISION

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/textproc/mandoc/Makefile,v
retrieving revision 1.8
diff -u -p -r1.8 Makefile
--- Makefile	22 Aug 2018 09:46:47 -0000	1.8
+++ Makefile	10 Oct 2018 23:14:51 -0000
@@ -1,7 +1,7 @@
 # $NetBSD: Makefile,v 1.8 2018/08/22 09:46:47 wiz Exp $
 
 DISTNAME=	mandoc-1.14.4
-PKGREVISION=	1
+PKGREVISION=	2
 CATEGORIES=	textproc devel
 MASTER_SITES=	http://mdocml.bsd.lv/snapshots/
 
@@ -18,6 +18,8 @@ SUBST_FILES.roff=	mandoc.1 mandoc.3
 SUBST_FILES.roff+=	eqn.7 man.7 mdoc.7 tbl.7
 SUBST_SED.roff=		-e 's,Xr roff,Xr mandoc-roff,g'
 
+CFLAGS.NetBSD+=	-D_OPENBSD_SOURCE
+
 # test needs perl
 USE_TOOLS+=	perl:build
 TEST_TARGET=	regress