Unset CFLAGS, CPPFLAGS and CXXFLAGS in build.rs

[fabricedesre]

If they are set to match a cross compilation target, they end up being used by the host compilation in build.rs . This doesn't end well :)

[jamienicol]

Happy to merge something like this, it's no worse than what we're already doing :). If anyone using this library actually needs to pass flags they could still use the HOST_CFLAGS var. Ideally one day rust-lang/cargo#5755 will remove the need for hacks like this.

I think we should be consistent in the variables which we ignore. So if gonk needs to ignore CPPFLAGS, lets also remove format!("CXXFLAGS_{}", &target) and format!("CPPFLAGS_{}", target.replace("-", "_"))

You could also avoid this issue by ensuring that any of CFLAGS_$HOST (eg CFLAGS_x86_64-unknown-linux-gnu), or the same with underscores instead of hyphens, or HOST_CFLAGS, are set, as those would take precedence over CFLAGS. Not sure if that would be easier for you?