build: fail in configure on missing dependency with --enable-bpf-compiler

The build of utils/nfbpf_compile depends on libpcap. If configure is
run with --enable-bpf-compiler, the script succeeds, but make fails.

This small patch adds a test for the dependency (libpcap) in configure
and fails hard if not found.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
diff --git a/configure.ac b/configure.ac
index 5ed5165..70f4eba 100644
--- a/configure.ac
+++ b/configure.ac
@@ -103,6 +103,10 @@
 AM_CONDITIONAL([ENABLE_LIBIPQ], [test "$enable_libipq" = "yes"])
 AM_CONDITIONAL([ENABLE_BPFC], [test "$enable_bpfc" = "yes"])
 
+if test "x$enable_bpfc" = "xyes"; then
+	AC_CHECK_LIB(pcap, pcap_compile,, AC_MSG_ERROR(missing libpcap library required by bpf compiler))
+fi
+
 PKG_CHECK_MODULES([libnfnetlink], [libnfnetlink >= 1.0],
 	[nfnetlink=1], [nfnetlink=0])
 AM_CONDITIONAL([HAVE_LIBNFNETLINK], [test "$nfnetlink" = 1])