selftests/bpf: use simply-expanded variables for libpcap flags

Save pkg-config output for libpcap as simply-expanded variables.
For an obscure reason 'shell' call in LDLIBS/CFLAGS recursively
expanded variables makes *.test.o files compilation non-parallel
when make is executed with -j option.

While at it, reuse 'pkg-config --cflags' call to define
-DTRAFFIC_MONITOR=1 option, it's exit status is the same as for
'pkg-config --exists'.

Fixes: f52403b6 ("selftests/bpf: Add traffic monitor functions.")
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240823194409.774815-1-eddyz87@gmail.comSigned-off-by: Martin KaFai Lau <martin.lau@kernel.org>

tools/testing/selftests/bpf/Makefile

@@ -48,9 +48,10 @@ CFLAGS += -g $(OPT_FLAGS) -rdynamic					\
 LDFLAGS += $(SAN_LDFLAGS)
 LDLIBS += $(LIBELF_LIBS) -lz -lrt -lpthread
 
-LDLIBS += $(shell $(PKG_CONFIG) --libs libpcap 2>/dev/null)
-CFLAGS += $(shell $(PKG_CONFIG) --cflags libpcap 2>/dev/null)
-CFLAGS += $(shell $(PKG_CONFIG) --exists libpcap 2>/dev/null && echo "-DTRAFFIC_MONITOR=1")
+PCAP_CFLAGS	:= $(shell $(PKG_CONFIG) --cflags libpcap 2>/dev/null && echo "-DTRAFFIC_MONITOR=1")
+PCAP_LIBS	:= $(shell $(PKG_CONFIG) --libs libpcap 2>/dev/null)
+LDLIBS += $(PCAP_LIBS)
+CFLAGS += $(PCAP_CFLAGS)
 
 # The following tests perform type punning and they may break strict
 # aliasing rules, which are exploited by both GCC and clang by default