tools build: Add a feature test for scandirat(), that is not implemented so far in musl and uclibc

We use it just when listing tracepoint events, and for root, so just
emit a warning about it to get users to ask the library maintainers to
implement it, as suggested in this systemd ticket:

https://github.com/systemd/casync/issues/129

Cc: Adrian Hunter <[e-mail]>
Cc: Ian Rogers <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: https://lore.kernel.org/lkml/[e-mail]/
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/build/Makefile.feature

···
         lzma                            \
         get_cpuid                       \
         bpf                             \
+        scandirat			\
         sched_getcpu			\
         sdt				\
         setns				\

tools/build/feature/Makefile

···
          test-gettid.bin			\
          test-jvmti.bin				\
          test-jvmti-cmlr.bin			\
+         test-scandirat.bin			\
          test-sched_getcpu.bin			\
          test-setns.bin				\
          test-libopencsd.bin			\
···
 	$(BUILD)
 
 $(OUTPUT)test-glibc.bin:
+	$(BUILD)
+
+$(OUTPUT)test-scandirat.bin:
 	$(BUILD)
 
 $(OUTPUT)test-sched_getcpu.bin:

tools/build/feature/test-all.c

···
 # include "test-pthread-barrier.c"
 #undef main
 
+#define main main_test_scandirat
+# include "test-scandirat.c"
+#undef main
+
 #define main main_test_sched_getcpu
 # include "test-sched_getcpu.c"
 #undef main
···
 	main_test_get_cpuid();
 	main_test_bpf();
 	main_test_libcrypto();
+	main_test_scandirat();
 	main_test_sched_getcpu();
 	main_test_sdt();
 	main_test_setns();

tools/build/feature/test-scandirat.c

···
+// SPDX-License-Identifier: GPL-2.0
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE
+#endif
+#include <dirent.h>
+
+int main(void)
+{
+	// expects non-NULL, arg3 is 'restrict' so "pointers" have to be different
+	return scandirat(/*dirfd=*/ 0, /*dirp=*/ (void *)1, /*namelist=*/ (void *)2, /*filter=*/ (void *)3, /*compar=*/ (void *)4);
+}
+
+#undef _GNU_SOURCE

tools/perf/Makefile.config

···
   NO_LIBDW_DWARF_UNWIND := 1
 endif
 
+ifeq ($(feature-scandirat), 1)
+  CFLAGS += -DHAVE_SCANDIRAT_SUPPORT
+endif
+
 ifeq ($(feature-sched_getcpu), 1)
   CFLAGS += -DHAVE_SCHED_GETCPU_SUPPORT
 endif