feat: pass sysroot when cross compiling

[charles-huet-ixxi]

pass sysroot when cross compiling

This is necessary when using a crate that will compile native libraries (in my case, sqlx with bundled sqlite), otherwise the native lib will not find system headers.

Also passing the cflags is necessary, as it may define the ABI (e.g. hard float on arm)i

[jschwe]

Also passing the cflags is necessary, as it may define the ABI (e.g. hard float on arm)

There are also CXX_FLAGS, $<LANG>_FLAGS_$<CONFIG> and c flags set as target properties.
If we start setting flags we should be consistent and think it through properly.

pass sysroot when cross compiling

It should be fine, but I'm a bit worried that this might break something in a different usecase. Corrosion doesn't really have a good test coverage of more complex projects.

For the moment I would lean towards not adding this to corrosion, and perhaps just encouraging users (by improving the documentation) to use corrosion_set_env_vars to set any required c-flags for cc-rs themselves.