rtla: Add optional dependency on BPF tooling

If tooling required for building BPF CO-RE skeletons is present (that
is, libbpf, clang with BPF CO-RE support, and bpftool), turn on
HAVE_BPF_SKEL flag.

Those requirements are similar to what perf requires, with the
difference of using system libbpf and bpftool instead of in-tree
versions.

rtla can be forcefully built without BPF skeleton support by setting
BUILD_BPF_SKEL=0 manually; in that case, a warning is displayed.

Cc: John Kacur <[e-mail]>
Cc: Luis Goncalves <[e-mail]>
Cc: Gabriele Monaco <[e-mail]>
Cc: Clark Williams <[e-mail]>
Link: https://lore.kernel.org/[e-mail]
Signed-off-by: Tomas Glozar <[e-mail]>
Signed-off-by: Steven Rostedt (Google) <[e-mail]>

tools/tracing/rtla/Makefile

···
 FEATURE_TESTS	:= libtraceevent
 FEATURE_TESTS	+= libtracefs
 FEATURE_TESTS	+= libcpupower
+FEATURE_TESTS	+= libbpf
+FEATURE_TESTS	+= clang-bpf-co-re
+FEATURE_TESTS	+= bpftool-skeletons
 FEATURE_DISPLAY	:= libtraceevent
 FEATURE_DISPLAY	+= libtracefs
 FEATURE_DISPLAY	+= libcpupower
+FEATURE_DISPLAY	+= libbpf
+FEATURE_DISPLAY	+= clang-bpf-co-re
+FEATURE_DISPLAY	+= bpftool-skeletons
 
 ifeq ($(V),1)
   Q		=

tools/tracing/rtla/Makefile.config

···
   $(info Please install libcpupower-dev/kernel-tools-libs-devel)
 endif
 
+ifndef BUILD_BPF_SKEL
+  # BPF skeletons are used to implement improved sample collection, enable
+  # them by default.
+  BUILD_BPF_SKEL := 1
+endif
+
+ifeq ($(BUILD_BPF_SKEL),0)
+  $(info BPF skeleton support disabled, building without BPF skeleton support.)
+endif
+
+$(call feature_check,libbpf)
+ifeq ($(feature-libbpf), 1)
+  $(call detected,CONFIG_LIBBPF)
+else
+  $(info libbpf is missing, building without BPF skeleton support.)
+  $(info Please install libbpf-dev/libbpf-devel)
+  BUILD_BPF_SKEL := 0
+endif
+
+$(call feature_check,clang-bpf-co-re)
+ifeq ($(feature-clang-bpf-co-re), 1)
+  $(call detected,CONFIG_CLANG_BPF_CO_RE)
+else
+  $(info clang is missing or does not support BPF CO-RE, building without BPF skeleton support.)
+  $(info Please install clang)
+  BUILD_BPF_SKEL := 0
+endif
+
+$(call feature_check,bpftool-skeletons)
+ifeq ($(feature-bpftool-skeletons), 1)
+  $(call detected,CONFIG_BPFTOOL_SKELETONS)
+else
+  $(info bpftool is missing or not supporting skeletons, building without BPF skeleton support.)
+  $(info Please install bpftool)
+  BUILD_BPF_SKEL := 0
+endif
+
+ifeq ($(BUILD_BPF_SKEL),1)
+  CFLAGS += -DHAVE_BPF_SKEL
+  EXTLIBS += -lbpf
+endif
+
 ifeq ($(STOP_ERROR),1)
   $(error Please, check the errors above.)
 endif