perf tools: Fix LIBELF_MMAP checking

Currently checking mmap support in libelf failed due to wrong flags.

    CHK libelf
    CHK libdw
    CHK libunwind
    CHK -DLIBELF_MMAP
/tmp/ccYJwdR0.o: In function `main':
:(.text+0x18): undefined reference to `elf_begin'
collect2: error: ld returned 1 exit status

This cannot happen since we checked the elf_begin() when checking
libelf and it succeeded.

Fix it by using a same flag with libelf checking.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Borislav Petkov <bp@amd64.org>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1351241752-2919-5-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>