musl doesn't have strtonum, import it from OpenBSD.

diff -upr docbook2mdoc-1.1.0.orig/Makefile docbook2mdoc-1.1.0/Makefile
--- docbook2mdoc-1.1.0.orig/Makefile	2020-12-29 10:20:53.933977196 +0100
+++ docbook2mdoc-1.1.0/Makefile	2020-12-29 10:21:24.500779954 +0100
@@ -4,8 +4,8 @@ WWWPREFIX = /var/www/vhosts/mdocml.bsd.l
 PREFIX = /usr/local
 
 HEADS =	xmalloc.h node.h parse.h reorg.h macro.h format.h
-SRCS =	xmalloc.c node.c parse.c reorg.c macro.c docbook2mdoc.c tree.c main.c
-OBJS =	xmalloc.o node.o parse.o reorg.o macro.o docbook2mdoc.o tree.o main.o
+SRCS =	xmalloc.c node.c parse.c reorg.c macro.c docbook2mdoc.c tree.c main.c strtonum.c
+OBJS =	xmalloc.o node.o parse.o reorg.o macro.o docbook2mdoc.o tree.o main.o strtonum.o
 DISTFILES = Makefile NEWS docbook2mdoc.1
 
 all: docbook2mdoc
diff -upr docbook2mdoc-1.1.0.orig/strtonum.c docbook2mdoc-1.1.0/strtonum.c
--- docbook2mdoc-1.1.0.orig/strtonum.c	2020-12-29 10:22:06.460966781 +0100
+++ docbook2mdoc-1.1.0/strtonum.c	2020-12-29 10:21:09.747380938 +0100
@@ -0,0 +1,66 @@
+/*	$OpenBSD: strtonum.c,v 1.8 2015/09/13 08:31:48 guenther Exp $	*/
+
+/*
+ * Copyright (c) 2004 Ted Unangst and Todd Miller
+ * All rights reserved.
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <errno.h>
+#include <limits.h>
+#include <stdlib.h>
+
+#define	INVALID		1
+#define	TOOSMALL	2
+#define	TOOLARGE	3
+
+long long
+strtonum(const char *numstr, long long minval, long long maxval,
+    const char **errstrp)
+{
+	long long ll = 0;
+	int error = 0;
+	char *ep;
+	struct errval {
+		const char *errstr;
+		int err;
+	} ev[4] = {
+		{ NULL,		0 },
+		{ "invalid",	EINVAL },
+		{ "too small",	ERANGE },
+		{ "too large",	ERANGE },
+	};
+
+	ev[0].err = errno;
+	errno = 0;
+	if (minval > maxval) {
+		error = INVALID;
+	} else {
+		ll = strtoll(numstr, &ep, 10);
+		if (numstr == ep || *ep != '\0')
+			error = INVALID;
+		else if ((ll == LLONG_MIN && errno == ERANGE) || ll < minval)
+			error = TOOSMALL;
+		else if ((ll == LLONG_MAX && errno == ERANGE) || ll > maxval)
+			error = TOOLARGE;
+	}
+	if (errstrp != NULL)
+		*errstrp = ev[error].errstr;
+	errno = ev[error].err;
+	if (error)
+		ll = 0;
+
+	return (ll);
+}
+DEF_WEAK(strtonum);