selftests/bpf: Support dynamically linking LLVM if static is not avai…

…lable

Since 67ab80a ("selftests/bpf: Prefer static linking for LLVM
libraries"), only statically linking test_progs is supported. However,
some distros only provide a dynamically linkable LLVM.

This commit adds a fallback for dynamically linking LLVM if static
linking is not available. If both options are available, static linking
is chosen.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Tested-by: Eduard Zingerman <eddyz87@gmail.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/bpf/872b64e93de9a6cd6a7a10e6a5c5e7893704f743.1738276344.git.dxu@dxuuu.xyz

tools/testing/selftests/bpf/Makefile

@@ -184,9 +184,14 @@ ifeq ($(feature-llvm),1)
   LLVM_CONFIG_LIB_COMPONENTS := mcdisassembler all-targets
   # both llvm-config and lib.mk add -D_GNU_SOURCE, which ends up as conflict
   LLVM_CFLAGS  += $(filter-out -D_GNU_SOURCE,$(shell $(LLVM_CONFIG) --cflags))
-  LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --libs $(LLVM_CONFIG_LIB_COMPONENTS))
-  LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
-  LLVM_LDLIBS  += -lstdc++
+  # Prefer linking statically if it's available, otherwise fallback to shared
+  ifeq ($(shell $(LLVM_CONFIG) --link-static --libs &> /dev/null && echo static),static)
+    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --libs $(LLVM_CONFIG_LIB_COMPONENTS))
+    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-static --system-libs $(LLVM_CONFIG_LIB_COMPONENTS))
+    LLVM_LDLIBS  += -lstdc++
+  else
+    LLVM_LDLIBS  += $(shell $(LLVM_CONFIG) --link-shared --libs $(LLVM_CONFIG_LIB_COMPONENTS))
+  endif
   LLVM_LDFLAGS += $(shell $(LLVM_CONFIG) --ldflags)
 endif