[zlib-bench] Let a leading '-' be enough to indicate options

Look for one '-' instead of two '--', to process program options. Some
users have old habits like typing -? for help, for example.

Bug: 798943
Change-Id: I6d757b161ffb2082c8ad0a96e41a31f479969a30
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2160955Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Leon Scroggins <scroggo@google.com>
Auto-Submit: Noel Gordon <noel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#761958}

third_party/zlib/contrib/bench/zlib_bench.cc

@@ -325,7 +325,7 @@ int main(int argc, char* argv[]) {
   else
     usage_exit(argv[0]);
 
-  while (argn < argc && !strncmp(argv[argn], "--", 2)) {
+  while (argn < argc && argv[argn][0] == '-') {
     if (get_option(argc, argv, "--compression")) {
       if (!get_compression(argc, argv, &zlib_compression_level))
         usage_exit(argv[0]);