Remove a TODO comment that was already done.
Update the build system to define what target platform it builds for.

This can then be used to run platform specific code.
On Linux, use cout instead of printf in grep to enjoy better performance.

On linux this gives us about 30% performance gain on
  $ time jrep -R struct linux-3.12.6

On MacOS using cout gives a huge performance penalty. The above command becomes
about four times slower.
Some quick profiling shows that when using printf we spend about 30% of the time
in printf, but when using cout, about 80% is spent in output operations, most of
it in lock related operations.