Windows Builds Fail when clang-cl Flags are added to CXXFLAGS

[Redfire75369]

The root issue here is that bindgen uses clang (not clang-cl) on windows, and on unrecognised flags, it fails. Upon adding CXXFLAGS="/clang:-flto=thin /clang:-fuse-ld=lld-link", the below code automatically adds it to bindgen's flags as well. As /clang: flags are only recognised by clang-cl, there is no way for cc and the makefile.cargo to receive the CXXFLAGS but not bindgen.

mozjs/mozjs-sys/build.rs

Lines 334 to 338 in c7fb1b8

	if let Ok(flags) = env::var("CXXFLAGS") {
	for flag in flags.split_whitespace() {
	builder = builder.clang_arg(flag);
	}
	}

There are a few possible approaches:

1. If a flag starts with /clang: on windows, do not add it to bindgen.
2. Add a separate BINDGEN_CXXFLAGS variable for bindgen's flags.

As said earlier, the root issue is the usage of clang, which means this error may not be just from /clang: flags but almost any cl flag. Hence, I would suggest the latter approach.

For a concrete example of the failure, see https://github.com/Redfire75369/spiderfire/actions/runs/7112793006/job/19363443030#step:6:18053, where a libclang error is caused.

In the event it matters for updating whatever depends on it, #136 was the original PR where this behaviour was introduced.

[jschwe]

Does setting the CLANG_PATH environment variable to clang-cl fix this issue?