rustc: Respect CC variable at runtime

[richo]

This really should be passed in either from sniffing CFG_CC during
configure, or with a --cc-program flag to the configure script, but an
escape hatch is required for platforms with nothing called `cc' on PATH.

[luqmana]

You can already override the default with the -C linker=foo flag so I don't think we need another way to do it.

[richo]

Plausible. I think that docs in this area are definitely lacking though (this is the second time I've gone digging in rustc to implement an escape hatch for something I couldn't find the docs for)

[alexcrichton]

I'm also a little hesitant to use this as we one day may move to something like LLVM's linker or to invoking ld directly in which case running gcc or using CC may not make as much sense.

[richo]

tbc, the only reason I didn't close this is because I think the current system isn't very sustainable; not because I think this is necessarily a good/the right solution.

My biggest gripe is that the underlying mechanism is very opaque- and as end user trying to set CC in the controlling environment made a lot more sense than -C linker=foo which is buried only in the codegen options(?)

[steveklabnik]

So, what's the call here?

[alexcrichton]

@richo for now I think we're going to want to keep -C linker as is for now in order to keep the interface of the compiler under control. Note that cross compilation is a little tougher with general environment variables like CC leading to problems such as needing many environment variables.

For now I'm going to close this to help clear out @bors's queue, but we can certainly revisit this in the future if necessary!