Log Message:
Update bootstrap-mk-files to 20150831.  Changes:

  * Reduce unnecessary diffs in Darwin, Linux, and SunOS mods.

  * Sync WARNS flags on SunOS.

  * Remove obsolete modifications to bsd.sys.mk and sys.mk on Darwin.  This
    primarily brings in PROG_CXX support.

Tested in bulk builds on all modified platforms.