build.zig: Warn if "config.h" cannot be located

[topolarity]

Adds a warning if config.h could not be located:

$ ./stage2/bin/zig build -Dlog -Denable-llvm --zig-lib-dir lib --prefix $(pwd)/stage3 --search-prefix ~/local/llvm13-release
warning: config.h could not be located automatically. Consider providing it explicitly via "-Dconfig_h"

The search for "config.h" now also stops early if it sees a .git folder, so that we don't go beyond the repo root.

[Vexu]

Previous discussion: #11501 (review)

[topolarity]

Thanks for the context @Vexu - I'll have to rework this one, clearly.

I wonder if copying the config.h into the installation during the build would be acceptable? This way Zig would inherit its build settings from the Zig building it.

[topolarity]

Added a command to copy "config.h" to the output directory during installation. The intention is that a build of Zig should bootstrap itself with the build configuration used to build it, by default.

We may not want "config.h" in the install dir, though. In that case, I'll have to go back to the drawing board.

[andrewrk]

I don't really think config.h belongs in the installation of Zig. Although I admit it's mostly personal taste, if you produce an installation of Zig using Zig without going through stage1, then no config.h is produced. It seems like, therefore, a canonical installation of Zig does not include config.h.

Is there something wrong with the recommended way to handle stage1, stage2, and stage3 installations side-by-side by running commands directly from the cmake build directory, or by passing -Dconfig_h with the zig build commands?

[topolarity]

I share your preference actually 👌

Is there something wrong with the recommended way to handle stage1, stage2, and stage3 installations side-by-side by running commands directly from the cmake build directory, or by passing -Dconfig_h with the zig build commands?

I think this behavior is perfectly fine with the warning added. Let me scale back the change to just that.