perf build: Gracefully fail the build if BUILD_BPF_SKEL=1 is specified and clang isn't available

Build BPF skels require having a compiler able to generate BPF bytecode,
and so far this is only possible with clang, so check for its
availability and fail the build when the user explicitely ask for BPF
skels to be built.

Acked-by: Namhyung Kim <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Andrii Nakryiko <[email protected]>,
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Song Liu <[email protected]>
Yang: Yang Jihong <[email protected]>,
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

Author: acmel

tools/perf/Makefile.config

@@ -216,6 +216,12 @@ ifeq ($(call get-executable,$(BISON)),)
   dummy := $(error Error: $(BISON) is missing on this system, please install it)
 endif
 
+ifeq ($(BUILD_BPF_SKEL),1)
+  ifeq ($(call get-executable,$(CLANG)),)
+    dummy := $(error $(CLANG) is missing on this system, please install it to be able to build with BUILD_BPF_SKEL=1)
+  endif
+endif
+
 ifneq ($(OUTPUT),)
   ifeq ($(shell expr $(shell $(BISON) --version | grep bison | sed -e 's/.\+ \([0-9]\+\).\([0-9]\+\).\([0-9]\+\)/\1\2\3/g') \>\= 371), 1)
     BISON_FILE_PREFIX_MAP := --file-prefix-map=$(OUTPUT)=