Report error on missing values for addConfigHeader

[MrDmitry]

Observed behavior:

> ls
build.zig  foo.h.in

> cat foo.h.in
#define a @b@

> cat build.zig
const std = @import("std");

pub fn build(b: *std.Build) void {
    const out_h = b.addConfigHeader(.{
        .style = .{ .cmake = b.path("foo.h.in") },
    }, .{});
    const install_file = b.addInstallFileWithDir(out_h.getOutput(), .header, "foo.h");
    b.getInstallStep().dependOn(&install_file.step);
}

> zig build
install
└─ install generated to foo.h
   └─ configure cmake header foo.h.in to foo.h failure
error: /home/dmitry/zig/20230/foo.h.in:1: unable to substitute variable: error: MissingValue
error: HeaderConfigFailed
Build Summary: 0/3 steps succeeded; 1 failed (disable with --summary none)
install transitive failure
└─ install generated to foo.h transitive failure
   └─ configure cmake header foo.h.in to foo.h failure
error: the following build command failed with exit code 1:
...

The output does not exactly match the expected behavior from #20230, but it is consistent with how other ConfigHeader functions propagate errors.

Tests are updated to match the new functionality.
Standalone tests are modified to stay consistent with CMake behavior. Since this new behavior is different, I removed the conflicting cases.

Closes #20230

[andrewrk]

Well, it's certainly an improvement. It would be a lot more useful if it would tell you the names of the missing names though. Thanks!

[MrDmitry]

Yeah, I'm experimenting with reporting diagnostics. I should have something in the coming weeks.