configure.ac: use pkg-config to find netfilter_conntrack

Use pkg-config to retrieve the dependencies of netfilter_conntrack
and avoid the following build failure when building statically:

configure:13096: /data/buildroot-test/instance-1/output-1/host/bin/x86_64-linux-gcc -o conftest -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -Os   -static -I/data/buildroot-test/instance-1/output-1/host/x86_64-buildroot-linux-uclibc/sysroot/usr/include -DPCRE_STATIC -pthread -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64  -static -L/data/buildroot-test/instance-1/output-1/host/x86_64-buildroot-linux-uclibc/sysroot/usr/lib conftest.c -lnetfilter_conntrack -lnfnetlink -lev  -lcares -lsodium -lmbedcrypto -lpcre  >&5
/data/buildroot-test/instance-1/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-uclibc/8.3.0/../../../../x86_64-buildroot-linux-uclibc/bin/ld: /data/buildroot-test/instance-1/output-1/host/x86_64-buildroot-linux-uclibc/sysroot/usr/lib/libnetfilter_conntrack.a(api.o): in function `nfct_fill_hdr.constprop.4':
api.c:(.text+0x3f): undefined reference to `mnl_nlmsg_put_header'

Fixes:
 - http://autobuild.buildroot.org/results/6cad497a7ab941a0ee3fd7007defc81e30cdcbe0

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>

configure.ac

@@ -252,7 +252,8 @@ AC_ARG_ENABLE(connmarktos,
 if test x"$enable_connmarktos" = "xyes" ; then
 	AC_MSG_NOTICE([Linux Netfilter Conntrack support requested by --enable-connmarktos: ${enable_connmarktos}])
 	if test "x$enable_connmarktos" != "xno"; then
-	    AC_SEARCH_LIBS([nfct_query], [netfilter_conntrack],,[
+	    PKG_CHECK_MODULES([NETFILTER_CONNTRACK], [libnetfilter_conntrack],,
+	    [AC_SEARCH_LIBS([nfct_query], [netfilter_conntrack],,[
 	        if test x"$enable_connmarktos" = "xyes"; then
 	            AC_MSG_ERROR([--enable-connmarktos specified but libnetfilter-conntrack library not found])
 	        fi
@@ -262,7 +263,7 @@ if test x"$enable_connmarktos" = "xyes" ; then
 	        if test x"$enable_connmarktos" = "xyes"; then
 	            AC_MSG_ERROR([--enable-connmarktos specified but libnetfilter-conntrack headers not found])
 	        fi
-	        with_netfilter_conntrack=no])
+	        with_netfilter_conntrack=no])])
 	  # If nothing is broken; enable the libraries usage.
 	  if test "x$with_netfilter_conntrack" != "xno"; then
 		with_netfilter_conntrack=yes

src/Makefile.am

@@ -9,7 +9,7 @@ AM_CFLAGS += -I$(top_srcdir)/libcork/include
 endif
 AM_CFLAGS += $(LIBPCRE_CFLAGS)
 
-SS_COMMON_LIBS = $(INET_NTOP_LIB) $(LIBPCRE_LIBS)
+SS_COMMON_LIBS = $(INET_NTOP_LIB) $(LIBPCRE_LIBS) $(NETFILTER_CONNTRACK_LIBS)
 if !USE_SYSTEM_SHARED_LIB
 SS_COMMON_LIBS += $(top_builddir)/libbloom/libbloom.la \
                   $(top_builddir)/libipset/libipset.la \