sbin: build ping if at least one of INET & INET6 is enabled

It does not build (and serves no purpose) if neither is true (i.e.,
building WITHOUT_INET and WITHOUT_INET6).  Also add an explicit error
in ping to make this case clear.

PR:		260082
Sponsored by:	The FreeBSD Foundation

sbin/Makefile

@@ -53,7 +53,6 @@ SUBDIR=adjkerntz \
 	nfsiod \
 	nos-tun \
 	pfilctl \
-	ping \
 	rcorder \
 	reboot \
 	recoverdisk \
@@ -68,6 +67,10 @@ SUBDIR=adjkerntz \
 	tunefs \
 	umount
 
+.if ${MK_INET} != "no" || ${MK_INET6} != "no"
+SUBDIR+=	ping
+.endif
+
 SUBDIR.${MK_CCD}+=	ccdconfig
 SUBDIR.${MK_CXX}+=	devd
 SUBDIR.${MK_HAST}+=	hastctl

sbin/ping/main.c

@@ -57,6 +57,8 @@ __FBSDID("$FreeBSD$");
 #define	OPTSTR PING4OPTS
 #elif defined(INET6)
 #define	OPTSTR PING6OPTS
+#else
+#error At least one of INET and INET6 is required
 #endif
 
 int