Robust bindgen support #140

The previous approach to use `--sysroot` was pretty much incomplete, as zig uses multiple include directories to reduce the bundle size. This commit implements a future-proof strategy to determine correct compile options by using `zig cc` itself, which is also what bindgen (transitively via clang-sys) does internally. Also fixes a couple of minor issues: - Handles all three kinds of `BINDGEN_EXTRA_CLANG_ARGS` environment variable correctly (one of them was missing previously). - Existing `BINDGEN_EXTRA_CLANG_ARGS` no longer blocks bindgen support. Collected options are instead appended to them.

Zstd-rs didn't include enough headers and the incomplete bindgen support was simply overlooked. For example, `stdlib.h` didn't work.

Bindgen should have accepted a correct TARGET environment variable, so it makes sense to set only target-specific environment variables. This also fixes a small bug when only the pan-target variable is set, in which case the newly set target-specific variable should first inherit the pan-target value and then more options should have been appended, but the pan-target value was discarded previously.

Only the master version of Zig recognizes it, all existing releases don't and adds proper include directories only when a file with a corresponding extension is added. Note: this will write additional empty files to the cache directory, which should be writable (otherwise everything else will fail anyway). Since the cache directory may end up being cwd, this commit tried to use a specific enough file name to avoid collision.

This is specific to Zig because while Zig ships with the recent enough glibc it dynamically sets `__GLIBC_MINOR__` according to the target. At this point it seems more robust to collect *all* options from `zig cc` output, but this is not as viable because all we can see is cc1 options (`-###` or `-v`), not clang driver options from Zig. It seems that we have no direct means to collect the latter, so we have to be conservative about which options to collect.

It turned out that unused argument warnings (`-Wunused-command-line- argument`) come from cc1, not from driver, and as long as all options are "claimed"---not necessarily used---it doesn't issue warnings. On the other hands, Zig internally adds `-nostdinc` but we didn't because we wanted header-only C++ libraries in the host to be usable, but it turned out that the presence of `__has_include` made this horribly fragile (e.g. some versions of libc++ used `__has_include( <pthread.h>)` to determine pthread support), so we gave up. If this facility is really desired, consider using `-idirafter /usr/include`.

The latter is also a recent addition, and `-nostdinc` already implies `nostdinc++` so we no longer need guarantees from `-stdlib++-isystem`. (We can't still replace duplicate `-c-isystem` and `-cxx-isystem` into a single `-isystem` due to the relocation.)

This in particular affects `__has_include` new in C++17, which is also how the test internally works. Tested files are chosen so that there is no chance that other OSes have matching header files by accident.

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Robust bindgen support #140

Robust bindgen support #140

Commits on Jun 17, 2023