selftests: netfilter: fix libmnl pkg-config usage

1. Don't hard-code pkg-config
2. Remove distro-specific default for CFLAGS
3. Use pkg-config for LDLIBS

Fixes: a50a88f0 ("selftests: netfilter: fix a build error on openSUSE")
Suggested-by: Jan Engelhardt <jengelh@inai.de>
Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

tools/testing/selftests/netfilter/Makefile

@@ -8,8 +8,11 @@ TEST_PROGS := nft_trans_stress.sh nft_fib.sh nft_nat.sh bridge_brouter.sh \
 	ipip-conntrack-mtu.sh conntrack_tcp_unreplied.sh \
 	conntrack_vrf.sh nft_synproxy.sh rpath.sh
 
-CFLAGS += $(shell pkg-config --cflags libmnl 2>/dev/null || echo "-I/usr/include/libmnl")
-LDLIBS = -lmnl
+HOSTPKG_CONFIG := pkg-config
+
+CFLAGS += $(shell $(HOSTPKG_CONFIG) --cflags libmnl 2>/dev/null)
+LDLIBS += $(shell $(HOSTPKG_CONFIG) --libs libmnl 2>/dev/null || echo -lmnl)
+
 TEST_GEN_FILES =  nf-queue connect_close
 
 include ../lib.mk