Windows is the only modern OS that lacks any kind of a built-in diff

tool, making usage on Windows less convenient when working with SCMs

that don't provide their own native diff tool.
We've historically recommended a particular installer for GNU Diff on

Windows, but it's very old, and it's long past time we move away from

it. While there's no standalone replacement available from a

third-party, we have a few options.
Git for Windows includes a version of GNU Diff that can be run, though

it's not in the Git path unless running a Git-specific shell. Unity for

Windows also bundles a copy of GNU Diff. These are now explicitly in the

search path.
We're also going to be supplying our own copy with the RBTools for

Windows installer, which will be the first one we'll check.
The old legacy GnuWin32 version is still supported as a fallback, if

available. We now search the path directly, rather than requiring the

user to include it in the system search path.
And finally, for all platforms, we now accept gdiff as a GNU Diff

candidate, not just on SunOS. This is useful for macOS, BSD, and

Windows.

Unit tests pass.
Ran this on Windows, with the code tweaked to check all candidates

even after a match, and verified the results for all paths, depending

on what I had installed.