Add cotire

[pfultz2]

This can help speed up building in many targets.

[icarus-sparry]

At least a reference to where to find cotire would help, e.g. cotire

According to the page.

The functionality provided by cotire has been superseded by features added to CMake 3.16. Support for pre-compiling and 
unity builds is now built into CMake. Thus, there will not be any further updates or support for this project.