Source file level compile args

[steplee]

CMake allows setting compile args on specific source files. This is very useful in some circumstances. For example, I have a case where I want a file using a JSON header-only library to be optimized with -O3 and my other files to be optimized according to the debug mode. Or I have a huge file I don't want debug info for. Or I have a couple of ugly defines intended for a specific source file that can otherwise be hidden from other compile commands.

I think the closest thing in meson would be to split off a static lib with the files you want to specialize on, and provide the flags only to it. But this has the downside of requiring more targets.