samples: bpf: don't run probes at the local make stage

Quentin reports that commit 07c3bbd ("samples: bpf: print
a warning about headers_install") is producing the false
positive when make is invoked locally, from the samples/bpf/
directory.

When make is run locally it hits the "all" target, which
will recursively invoke make through the full build system.

Speed up the "local" run which doesn't actually build anything,
and avoid false positives by skipping all the probes if not in
kbuild environment (cover both the new warning and the BTF
probes).

Reported-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

samples/bpf/Makefile

@@ -206,6 +206,8 @@ HOSTCC = $(CROSS_COMPILE)gcc
 CLANG_ARCH_ARGS = -target $(ARCH)
 endif
 
+# Don't evaluate probes and warnings if we need to run make recursively
+ifneq ($(src),)
 HDR_PROBE := $(shell echo "\#include <linux/types.h>\n struct list_head { int a; }; int main() { return 0; }" | \
 	$(HOSTCC) $(KBUILD_HOSTCFLAGS) -x c - -o /dev/null 2>/dev/null && \
 	echo okay)
@@ -232,6 +234,7 @@ ifneq ($(and $(BTF_LLC_PROBE),$(BTF_PAHOLE_PROBE),$(BTF_OBJCOPY_PROBE)),)
 	DWARF2BTF = y
 endif
 endif
+endif
 
 # Trick to allow make to be run from this directory
 all: