Merge branch 'selftests-bpf-use-pkg-config-to-determine-ld-flags'

Akihiko Odaki says:

====================
selftests/bpf: Use pkg-config to determine ld flags

When linking statically, libraries may require other dependencies to be
included to ld flags. In particular, libelf may require libzstd. Use
pkg-config to determine such dependencies.

V4 -> V5: Introduced variables LIBELF_CFLAGS and LIBELF_LIBS.
          (Daniel Borkmann)
          Added patch "selftests/bpf: Choose pkg-config for the target".
V3 -> V4: Added "2> /dev/null".
V2 -> V3: Added missing "echo".
V1 -> V2: Implemented fallback, referring to HOSTPKG_CONFIG.
====================

Link: https://lore.kernel.org/r/20231125084253.85025-1-akihiko.odaki@daynix.com
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>