add check-commits target

This adds a 'check-commits' target to the build system. This is very useful for checking patch series, it will run through each invidividual commit on top of origin/master and execute the tests at each step.

The process can be customized through environment variables;

CHECK_BASE - the base object. defaults to 'origin/master'
CHECK_HEAD - the head object. defaults to 'HEAD'
CHECK_TARGET - the list of targets to build. defaults to 'all;test'. can be set to 'all' to just perform build tests.
CHECK_THREADS - number of threads to use when building (make -jX). defaults to number of logical cores on the system. might need some adjustments on OSX.

I always put this in all my build systems. And really miss it when i work on OPM. Hopefully we can scratch that itch now =)

Cosmetics, et al; i did not put much effort into those. just point out what you don't like, i'll gladly adjust.
there is no LINUX defined in CMake. there is however an APPLE. so you'd do

if(UNIX)
  if(APPLE)
  else()
  endif()
endif()

give me the code for the apple if() and i'll adjust.