Improve siege flags

Reviewed By: @​aravind

Differential Revision: D1548017
Make benchmarking script check HHVM build is suitable

Summary: ```
[fredemmott@devbig076 ~/fbcode/hphp/test/frameworks/perf] hhvm perf.php --toys --hhvm=/data/fio/fbcode/_bin/hphp/hhvm/hhvm
HHVM build is not suitable for benchmarking:
  HHVM build type: Debug
  Required: Release
Existing due to invalid config. You can run anyway with --i-am-not-benchmarking, but the results will not be suitable for any kind of comparison.
[fredemmott@devbig076 ~/fbcode/hphp/test/frameworks/perf] hhvm perf.php --toys --hhvm=/data/fio/fbcode/_bin/hphp/hhvm/hhvm --i-am-not-benchmarking

***********************
** 2014-09-09 21:40:42
** Installing framework

**********************
** 2014-09-09 21:40:42
** Starting Nginx
```

Reviewed By: @JoelMarcey

Differential Revision: D1545572
--wrap is specific to linux's gcc linker

Summary: This broke the Mac OSX build.

Reviewed By: @JoelMarcey

Differential Revision: D1546684