mirror: Add probe for libelf-zstd feature

Add the probe to test whether we need to link against the zstd library
to use libelf in bpftool.

This should ideally have been merged before commit 67f1758
("bpftool: Link zstd lib required by libelf"), where we have the
Makefile use the actual feature, but I forgot to add the probe when
synchronising the repository.

Elfutils commit a5b07cdf9c49 adds support for ZSTD compression, first
shipped with elfutils 0.189. Distros now have it; for example, Ubuntu
22.04 ships version 0.186 (or 0.188 in backports), but Ubuntu 24.04 has
0.189 or 0.190 depending on the arch.

Link: https://sourceware.org/git/?p=elfutils.git;a=commit;h=a5b07cdf9c491fb7a4a16598c482c68b718f59b9
Signed-off-by: Quentin Monnet <[email protected]>

Author: qmonnet

src/Makefile.feature

@@ -34,9 +34,41 @@ feature-clang-bpf-co-re := \
   $(findstring 1,$(call detect,$(CLANG_BPF_CO_RE_PROBE_CMD)))
 endif # clang-bpf-co-re
 
+### feature-libelf-zstd
+
+# Define these unconditionally so we can also use the probe for feature-libbfd
+LIBELF_ZSTD_PROBE := '$(pound)include <libelf.h>\n'
+LIBELF_ZSTD_PROBE += 'int main(void) {'
+LIBELF_ZSTD_PROBE += '	elf_compress(0, ELFCOMPRESS_ZSTD, 0);'
+LIBELF_ZSTD_PROBE += '	return 0;'
+LIBELF_ZSTD_PROBE += '}'
+
+LIBELF_ZSTD_PROBE_CMD = printf '%b\n' $(LIBELF_ZSTD_PROBE) | \
+  $(CC) $(CFLAGS) -Wall -Werror -x c - -lelf -lz -lzstd -o - >/dev/null
+
+define libelf_zstd_build
+  $(call detect,$(LIBELF_ZSTD_PROBE_CMD))
+endef
+
+ifneq ($(findstring libelf-zstd,$(FEATURE_TESTS)),)
+$(call LOG,Probing: feature-libelf-zstd)
+feature-libelf-zstd := $(findstring 1, $(call libelf_zstd_build))
+endif # libelf-zstd
+
 ### feature-libbfd
 
 ifneq ($(findstring libbfd,$(FEATURE_TESTS)),)
+
+# Check whether feature-libbfd probe needs libzstd
+ifneq ($(findstring libelf-zstd,$(FEATURE_TESTS)),)
+  need_libzstd := $(feature-libelf-zstd)
+else
+  need_libzstd := $(findstring 1, $(call libelf_zstd_build))
+endif
+ifeq ($(need_libzstd),1)
+  LIBZSTD_FLAG := -lzstd
+endif
+
 LIBBFD_PROBE := '$(pound)include <bfd.h>\n'
 LIBBFD_PROBE += 'int main(void) {'
 LIBBFD_PROBE += '	bfd_demangle(0, 0, 0);'
@@ -51,15 +83,15 @@ endef
 
 $(call LOG,Probing: feature-libbfd)
 feature-libbfd := \
-  $(findstring 1,$(call libbfd_build,-lbfd -ldl))
+  $(findstring 1,$(call libbfd_build,-lbfd $(LIBZSTD_FLAG) -ldl))
 ifneq ($(feature-libbfd),1)
   $(call LOG,Probing: feature-libbfd-liberty)
   feature-libbfd-liberty := \
-    $(findstring 1,$(call libbfd_build,-lbfd -ldl -liberty))
+    $(findstring 1,$(call libbfd_build,-lbfd $(LIBZSTD_FLAG) -ldl -liberty))
   ifneq ($(feature-libbfd-liberty),1)
     $(call LOG,Probing: feature-libbfd-liberty-z)
     feature-libbfd-liberty-z := \
-      $(findstring 1,$(call libbfd_build,-lbfd -ldl -liberty -lz))
+      $(findstring 1,$(call libbfd_build,-lbfd $(LIBZSTD_FLAG) -ldl -liberty -lz))
   endif
 endif
 HAS_LIBBFD := $(findstring 1, \