Do not invasively check for BPF (Ethernet check queries presence
of /dev/bpfX): we know that FreeBSD has BPF and ports can be *built*
inside infrastructure that has no such /dev entries; like a jail
without devfs configured to unhide BPF devices.

The presence of the net/bpf.h is checked in configure earlier,
so sanity checks for the build environment will still be present.

Macro HAVE_BSD_BPF won't be defined after this modification,
but as of libdnet-1.13 it is not used at all inside libdnet's code.

FreeBSD PR: https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=238523
Origin: rea@FreeBSD.org
--- configure.ac.orig	2019-03-19 17:55:02 UTC
+++ configure.ac
@@ -203,7 +203,6 @@ AC_CHECK_FUNCS([gethostbyname gettimeofday memmove mem
 AC_REPLACE_FUNCS(err strlcat strlcpy strsep)
 
 dnl Checks for other system-specific jonks.
-AC_DNET_BSD_BPF
 AC_DNET_RAWIP_HOST_OFFLEN
 
 dnl Check for arp interface.
@@ -219,18 +218,7 @@ else
 	(exit 1); exit 1;
 fi
 
-dnl Check for Ethernet interface.
-if test "$ac_cv_dnet_bsd_bpf" = yes ; then
-	AC_LIBOBJ([eth])
-else
-	echo "|------------------------------------------|"
-	echo "|No suitable Ethernet interface found. Most|"
-	echo "|probably you are not running the FreeBSD  |"
-	echo "|OS. Please consider using this version    |"
-	echo "|of libdnet  with the FreeBSD system       |"
-	echo "|------------------------------------------|"
-	(exit 1); exit 1;
-fi
+AC_LIBOBJ([eth])
 
 dnl Check for firewall interface.
 if test "$ac_cv_header_net_pfvar_h" = yes ; then