Regression test portability: arequal-checksum

Building arequal-checksum on non Linux systems requires a few adjustments:
- use __builtin_alloca() on all platforms
- on systems without argp, get it from contrib/standalone-argp
  (this requires adding cflags support to the build_tester function)
- FTW_ACTIONRETVAL option to nftw(3) is GNU libc specific, work around
  if it does not exist.
- md5sum is Linux-specific. Use md5 -n on NetBSD and md5 -q on FreeBSD/Darwin
- Use 'cc' as synonymous for all compilers, it can behave as gcc/clang depending
  on which is default
- cleanup tabs/whitespaces

BUG: 764655
Change-Id: I9090c17da596fbf00fc1fbd7593163ce8cd3b84c
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Signed-off-by: Harshavardhana <harsha@harshavardhana.net>
Reviewed-on: http://review.gluster.org/8283
Tested-by: Gluster Build System <jenkins@build.gluster.com>