custom_build: add HOST env var

[codyps]

This is just to support some nice environment aliases in gcc-rs at the moment.

[alexcrichton]

Perhaps instead of BUILD_KIND a HOST environment variable could be exported? We would define HOST as the host triple of the rust compiler that we're using.

[codyps]

I suppose so. I'll end up extracting the same info from that by just doing ${HOST} == ${TARGET} ? "HOST" : "TARGET".

Are you thinking that it might be useful for some build scripts to examine the host?

[codyps]

I'd also like to note that testing this plus the cooperating gcc-rs patch seems to result in target rlibs containing host objects (causing the final link to fail). [Strictly, this is better than what happens without these changes, which is build failure in the first user of gcc-rs]

I'm looking at the cargo code to figure out why this is happening, any ideas?

[codyps]

I've updated to supply a HOST environment variable.

As an aside: supplying HOST rather than BUILD_KIND also makes bootstrapping a version of cargo with this merged easier, as one can manually set HOST and have it be correct for an entire cargo invocation.

[alexcrichton]

I was just thinking that it's more in line with TARGET to just say exactly what we're compiling for and you can derive information such as whether it's a cross compile or not from the general information.

I'm not quite sure what's going on with gcc-rs, could you elaborate a little more on that?

[codyps]

On the issue with cross-compilation and gcc-rs:

Heres a log

Relevant error should be selected, but I'll include it here for clarity:

target/arm-poky-linux-gnueabi/release/deps/libtime-da2a5e65e3acd945.rlib: error adding symbols: File format not recognized

That rlib actually contains a mix of TARGET (elf32 arm) and HOST (elf64 amd64) object files. Here's a readelf -h: https://gist.github.com/jmesmon/d64e0066decd7548f463

It appears that somehow the HOST output of the time crate's build.rs is getting mixed with the TARGET output of the time crate's rust code, which then fails to link.

[alexcrichton]

That is... surprising! I'd have to take some more time to investigate though.