build.zig: Try harder to find "config.h"

Now that we're building stage3 frequently, I noticed that stage2 can't
auto-locate config.h because it's not in its own (or a parent)
directory.

This change makes `build.zig` try a little harder to find the `config.h`
from CMake: It will check for "build"/"stage1" directories that might
contain the file. More importantly, it will warn the user if it could
not be located.

Author: topolarity

build.zig

@@ -683,17 +683,24 @@ fn findAndParseConfigH(b: *Builder, config_h_path_option: ?[]const u8) ?CMakeCon
             var dir = fs.cwd().openDir(check_dir, .{}) catch unreachable;
             defer dir.close();
 
-            break :blk dir.readFileAlloc(b.allocator, "config.h", max_config_h_bytes) catch |err| switch (err) {
-                error.FileNotFound => {
-                    const new_check_dir = fs.path.dirname(check_dir);
-                    if (new_check_dir == null or mem.eql(u8, new_check_dir.?, check_dir)) {
-                        return null;
-                    }
-                    check_dir = new_check_dir.?;
-                    continue;
-                },
-                else => unreachable,
-            };
+            for ([_]?[]const u8{ null, "build", "stage1" }) |build_dir| {
+                var sub_dir = dir;
+                if (build_dir) |d|
+                    sub_dir = dir.openDir(d, .{}) catch continue;
+
+                break :blk sub_dir.readFileAlloc(b.allocator, "config.h", max_config_h_bytes) catch |err| switch (err) {
+                    error.FileNotFound => continue,
+                    else => unreachable,
+                };
+            } else {
+                const new_check_dir = fs.path.dirname(check_dir);
+                if (new_check_dir == null or mem.eql(u8, new_check_dir.?, check_dir)) {
+                    std.log.warn("config.h could not be located automatically. Consider providing it explicitly via \"-Dconfig_h\"", .{});
+                    return null;
+                }
+                check_dir = new_check_dir.?;
+                continue;
+            }
         } else unreachable; // TODO should not need `else unreachable`.
     };