Support XC8 2.x

[Elemecca]

Currently only XC8 v1.x is supported, although v2.x works with the legacy command-line driver (see last paragraph).

In v2.x Microchip switched from their proprietary HI-TECH C frontend to Clang in order to support C99. Rather than implement PIC codegen in LLVM, though, they wrote a translator from LLVM's IR to p-code (the HI-TECH IR), which is then passed through the same code generator as the old HI-TECH frontend. XC8 still includes the HI-TECH C frontend to support older projects.

Some code (M-Stack, notably) that worked under HI-TECH C does not compile in Clang. Also, the LLVM IR translation currently produces larger and less efficient code than the IR generated by HI-TECH C. Many projects have declined to upgrade until some of the kinks are worked out. We will therefore have to support both the HI-TECH C and Clang frontends for the foreseeable future.

v2.x also introduces a new compiler command-line driver, xc8-cc, which uses an option format more in line with Clang and GCC. It supports both C frontends via the -std option: -std=c99 for Clang and -std=c90 or -std=c89 for HI-TECH C. The old command-line driver xc8 is still included. We will need to support both command-line drivers, both so v1.x still works and to support projects already using the v1.x driver who want to upgrade to v2.x without changing their compiler options.

[Elemecca]

It looks like @enbyted has already done some work on this in enbyted@f688dc8

I'm not going to merge that directly as it pushes users to use xc8-cc more aggressively than I'm comfortable with and doesn't provide a way to select the command-line driver, C frontend, or C standard.

I also want to see if I can get CMake to detect xc8-cc as Clang and avoid having to write fully custom compiler support for it.

[Elemecca]

So apparently xc8-cc, despite being a frontend to Clang, is entirely custom and uses mostly GCC-style options, probably for compatibility with XC16 and XC32. It might be possible to feed CMake pic/bin/clang and have it use its native Clang support, but that seems dangerous. It's probably best to just write custom compiler support.

[enbyted]

If you have an idea in mind how it should work I'd love to work on it as I'm using this CMake setup with all new projects currently.

It probably isn't feasible to use clang directly, as we'd have to accurately emulate all options set by xc8-cc. However in my limited usage I didn't encounter any command-line incompatibilities with GCC - maybe it would be possible to use slightly modified GCC compiler support?

I've never dug so deep in CMake, but hey, there's a lot to learn!