Allow per-target cmake toolchain

[roblabla]

This would help cross-compilation a bit, allowing the user to have some global environment variables set up for each of their cross-compilation toolchain.

The get_target_env_var function is taken from cc-rs::get_var function.

[alexcrichton]

Thanks! Could this be done for most other env var accesses as well? Additionally, like cc-rs, could this print out the results of what env vars are inspected (sometimes very helpful for debugging failures)

[roblabla]

Could this be done for most other env var accesses as well?

Right, so here are the two other environment variables where I think it'd make sense to make them per-target:

• CMAKE_GENERATOR: Makes sense, you may want to use mingw makefiles or msbuild depending on the target for instance.
• CMAKE_INSTALL_PREFIX: This is used by find_package and company.

The other two, I'm not so sure:

• DEP_{dep}_ROOT: I'm not sure what this does.
• CMAKE: This just points to the cmake binary. I don't think it makes sense to make it per-target, as I believe the same cmake binary is used when cross-compiling (at least that's what I do!).

[alexcrichton]

For DEP_* those are cargo-specific things so they should avoid this function (although it might be nice to print out that they're being accessed like cc-rs does), but for CMAKE I'd just throw it into the same function for consistency, but I agree that it's pretty doubtful anyone will use a target-specific variable for that.

[roblabla]

How does this look? It should now get the per-target environment variable for CMAKE, CMAKE_GENERATOR, CMAKE_PREFIX_PATH and CMAKE_TOOLCHAIN_FILE, and print on access. The same env_cache logic found in cc-rs was used here to make sure we only print once per env variable.

[alexcrichton]

Thanks!