cmd/go: race detector and cgo builds needs to diagnose lack of C compiler more gracefully

[dr2chase]

NB this is forwarded from a Stack Overflow question.
A new user had this experience:

I'm following a tutorial on Go, and I have the actual file tree:
.
├── arrays
├── concurrency
├── di
├── hello-world
├── integers
├── iteration
├── maps
├── mocking
├── pointers
├── racer
└── structs
If I run the tests in all of the folders files, they work, except for the folder called racer, where running the tests gives me the following error:

# runtime/cgo
cgo: exec /missing-cc: fork/exec /missing-cc: no such file or directory
FAIL    github.com/-redacted-/go-with-tests/racer [build failed]

Is it related to something I'm doing in the files?

I can't reproduce this without trashing my Xcode install which I'd rather not, but it is almost certainly the case that said/sad user lacks a C compiler, and building with the race detector requires a C compiler. If we could detect this situation and give a more helpful error message, that would probably improve the on-ramp experience.