[build.zig] Override config.h definitions (#4193)

* [build.zig] Overridable definitions from config.h
The new Options field "config" holds a string the user can set in the
format "-Dflag_a=1 -Dflag_b=0 ..." to override the values set in
`config.h`.
The file is parsed and the default values are appended to the
compilation flags, if the user doesn't override them.
The user string is appended to the compilation flags.
The "-DEXTERNAL_CONFIG_FLAGS" is added to prevent "config.h" inclusion.

Note: a certain format is assumed for the formatting of config.h
Note: this commit references the closed issue #3516

* [build.zig] Only SUPPORT_* definitions are overridable

Lines from `config.h` which contains "SUPPORT" are added to compilation after being parsed:
- remove whitespace
- format to preprocessor option https://gcc.gnu.org/onlinedocs/gcc/Preprocessor-Options.html

The user supplied flags have priority over the ones read from the file.

NOTE: extension to commit 4da7f82e6f912485351167da3bbc91807371fdee, the logic is simplified
because the SUPPORT flags only have binary values, which makes them easier to parse.

src/build.zig

@@ -28,6 +28,7 @@ pub fn addRaylib(b: *std.Build, target: std.Build.ResolvedTarget, optimize: std.
         .shared = options.shared,
         .linux_display_backend = options.linux_display_backend,
         .opengl_version = options.opengl_version,
+        .config = options.config,
     });
     const raylib = raylib_dep.artifact("raylib");
 
@@ -52,6 +53,36 @@ fn compileRaylib(b: *std.Build, target: std.Build.ResolvedTarget, optimize: std.
         "-DGL_SILENCE_DEPRECATION=199309L",
         "-fno-sanitize=undefined", // https://github.com/raysan5/raylib/issues/3674
     });
+    if (options.config) |config| {
+        const file = try std.fs.path.join(b.allocator, &.{ std.fs.path.dirname(@src().file) orelse ".", "config.h" });
+        defer b.allocator.free(file);
+        const content = try std.fs.cwd().readFileAlloc(b.allocator, file, std.math.maxInt(usize));
+        defer b.allocator.free(content);
+
+        var lines = std.mem.split(u8, content, "\n");
+        while (lines.next()) |line| {
+            if (!std.mem.containsAtLeast(u8, line, 1, "SUPPORT")) continue;
+            if (std.mem.startsWith(u8, line, "//")) continue;
+
+            var flag = std.mem.trimLeft(u8, line, " \t");                 // Trim whitespace
+            flag = flag["#define ".len - 1 ..];                           // Remove #define
+            flag = std.mem.trimLeft(u8, flag, " \t");                     // Trim whitespace
+            flag = flag[0..std.mem.indexOf(u8, flag, " ").?];             // Flag is only one word, so capture till space
+            flag = try std.fmt.allocPrint(b.allocator, "-D{s}", .{flag}); // Prepend with -D
+
+            // If user specifies the flag skip it
+            if (std.mem.containsAtLeast(u8, config, 1, flag)) continue;
+
+            // Append default value from config.h to compile flags
+            try raylib_flags_arr.append(b.allocator, flag);
+        }
+
+        // Append config flags supplied by user to compile flags
+        try raylib_flags_arr.append(b.allocator, config);
+
+        try raylib_flags_arr.append(b.allocator, "-DEXTERNAL_CONFIG_FLAGS");
+    }
+
     if (options.shared) {
         try raylib_flags_arr.appendSlice(b.allocator, shared_flags);
     }
@@ -253,6 +284,7 @@ pub const Options = struct {
     shared: bool = false,
     linux_display_backend: LinuxDisplayBackend = .Both,
     opengl_version: OpenglVersion = .auto,
+    config: ?[]const u8 = null,
 
     raygui_dependency_name: []const u8 = "raygui",
 };
@@ -307,6 +339,7 @@ pub fn build(b: *std.Build) !void {
         .shared = b.option(bool, "shared", "Compile as shared library") orelse defaults.shared,
         .linux_display_backend = b.option(LinuxDisplayBackend, "linux_display_backend", "Linux display backend to use") orelse defaults.linux_display_backend,
         .opengl_version = b.option(OpenglVersion, "opengl_version", "OpenGL version to use") orelse defaults.opengl_version,
+        .config = b.option([]const u8, "config", "Compile with custom define macros overriding config.h") orelse null,
     };
 
     const lib = try compileRaylib(b, target, optimize, options);