One approach for testing with multiple compilers that I use on some Fortran projects (where testing against multiple compilers seems more common than in C) is to use a variable from the command line to specify the compiler, for example:

    make FC=ifx check

I have made a script to run through a series of compilers by alternating between `make check` and `make clean`.

I have separate Makefiles for GNU Make and NMAKE/jom. My Fortran code works fine on various Linux distributions and Windows, though I'll add that achieving that is probably easier with Fortran than C. I've also tried a BSD Make that worked (on Ubuntu at least). My Makefiles are pretty close to the intersection of POSIX and NMAKE, so the main differences between the different Make versions are the conditional statements needed to handle the different compiler flags and the include statements (as I put the compiler flags in separate files).

And architectures. Probably a bunch of build servers or a swarm of docker, qemu, and VMs, with a good test coverage to detect behaviour differences.

In practice, the compiler is an often an omitted dependency of any c code.

Realistically, you don't aim to support every platform. You have a finite set of platforms you want to support. As the size of that set goes up, so does the probability it'll work on platforms unknown to you, but it's never 100%. Did you know some platforms have 32-bit char?