Fix toolchain paths on certain systems.

[umanwizard]

Certain systems, e.g. Guix, have e.g. $ARCH-unknown-linux-gnu-gcc rather than $ARCH-linux-gnu-gcc.

Thus, use the former as a fallback if the latter isn't found.

[florianl]

Looks good to me - just a nit.

[christos68k]

I think we don't use these when building the artifacts (as we dropped CGo), they're only used in tests where we explicitly enable CGo. In which case, shouldn't we just run the tests through the Docker image as well for reproducibility (and also avoiding adding environment-specific fallbacks)?

One issue with this implementation is that if no compiler is found, the errors read weird as CC is empty:

test.c -g -o with-debug-syms
make[1]: test.c: No such file or directory
make[1]: *** [Makefile:21: with-debug-syms] Error 127
make[1]: *** Waiting for unfinished jobs....
/bin/sh: 1: test.c: not found

[umanwizard]

They are also used when running make generate.

[christos68k]

They are also used when running make generate.

I missed this because make generate doesn't fail when the command -v invocations don't find a compiler (make test will however fail as shown in my previous comment):

$ (cd ./support; CC=$(command -v false) ./generate.sh)
$ (cd ./support; CC=$(command -v foo) ./generate.sh)
types_gen.go

strace is showing Go searching for and executing gcc when CC=. So essentially, there are multiple points of discussion here:

1. We want make to fail when no compiler matching our criteria is found instead of introducing indeterminism.
2. Rather than adding environment-specific fallbacks (Guix, Nix, ..), we should probably strive for a cleaner Makefile and strongly recommend people use our build container.