Check in initial unmodified cmake action as generated using the github UI.

This is the final result of experiments done in
#1 slightly modified to incorporate
suggestions from rizsotto in librsync#222.
The original experiments had a messy commit history including many dead-ends
so this cleaner patch was created rather than merge all that mess.

It includes testing on ubuntu, macos, and windows for Release builds with a
variety of cmake options, and a Debug build on ubuntu. The options include
testing with clang, Ninja, static libs, and excluding rdiff. It installs all
required packages using apt-get, brew, or vcpkg to fully build and test rdiff
on all platforms.

Install the ninja-build package on Linux for the test that uses it.