bpftool: Prevent setting duplicate _GNU_SOURCE in Makefile

bpftool: Prevent setting duplicate _GNU_SOURCE in Makefile

When building selftests with CFLAGS set via env variable, the value of
CFLAGS is propagated into bpftool Makefile (called from selftests
Makefile). This makes the compilation fail as _GNU_SOURCE is defined two
times - once from selftests Makefile (by including lib.mk) and once from
bpftool Makefile (by calling `llvm-config --cflags`):

    $ CFLAGS="" make -C tools/testing/selftests/bpf
    [...]
    CC      /bpf-next/tools/testing/selftests/bpf/tools/build/bpftool/btf.o
    <command-line>: error: "_GNU_SOURCE" redefined [-Werror]
    <command-line>: note: this is the location of the previous definition
    cc1: all warnings being treated as errors
    [...]

Filter out -D_GNU_SOURCE from the result of `llvm-config --cflags` in
bpftool Makefile to prevent this error.

Signed-off-by: Viktor Malik <vmalik@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Quentin Monnet <qmo@kernel.org>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/acec3108b62d4df1436cda777e58e93e033ac7a7.1730449390.git.vmalik@redhat.com