build.zig: check build/config.h for cmake config

Stage1 doesn't need -Dconfig_h to be specified since it'll find it
in the same directory as the executable but since stage2 binary is
in `zig-out/bin/zig` it'll not find `config.h` and silently default
to static LLVM and fail with a linker error.

Also looking for `build/config.h` makes building stage3 work with
the same command as building stage2.

Author: Vexu

build.zig

@@ -671,6 +671,13 @@ fn findAndParseConfigH(b: *Builder, config_h_path_option: ?[]const u8) ?CMakeCon
             var dir = fs.cwd().openDir(check_dir, .{}) catch unreachable;
             defer dir.close();
 
+            if (dir.readFileAlloc(b.allocator, "build/config.h", max_config_h_bytes)) |some| {
+                break :blk some;
+            } else |err| switch (err) {
+                error.FileNotFound => {},
+                else => unreachable,
+            }
+
             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);