Laura reported that the perf build failed in fedora when we got a glibc
that provides gettid(), which I reproduced using fedora rawhide with the
glibc-devel-2.29.9000-26.fc31.x86_64 package.

Add a feature check to avoid providing a gettid() helper in such
systems.

On a fedora rawhide system with this patch applied we now get:

  [root@7a5f55352234 perf]# grep gettid /tmp/build/perf/FEATURE-DUMP
  feature-gettid=1
  [root@7a5f55352234 perf]# cat /tmp/build/perf/feature/test-gettid.make.output
  [root@7a5f55352234 perf]# ldd /tmp/build/perf/feature/test-gettid.bin
          linux-vdso.so.1 (0x00007ffc6b1f6000)
          libc.so.6 => /lib64/libc.so.6 (0x00007f04e0a74000)
          /lib64/ld-linux-x86-64.so.2 (0x00007f04e0c47000)
  [root@7a5f55352234 perf]# nm /tmp/build/perf/feature/test-gettid.bin | grep -w gettid
                   U gettid@@GLIBC_2.30
  [root@7a5f55352234 perf]#

While on a fedora:29 system:

  [acme@quaco perf]$ grep gettid /tmp/build/perf/FEATURE-DUMP
  feature-gettid=0
  [acme@quaco perf]$ cat /tmp/build/perf/feature/test-gettid.make.output
  test-gettid.c: In function ‘main’:
  test-gettid.c:8:9: error: implicit declaration of function ‘gettid’; did you mean ‘getgid’? [-Werror=implicit-function-declaration]
    return gettid();
           ^~~~~~
           getgid
  cc1: all warnings being treated as errors
  [acme@quaco perf]$
Reported-by: Laura Abbott <labbott@redhat.com>
Tested-by: Laura Abbott <labbott@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: https://lkml.kernel.org/n/tip-yfy3ch53agmklwu9o7rlgf9c@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>