Simplify build.zig to not require user to specify raylib path (#2383)

We can figure out the source file location based on the location of the
build.zig file. No need to require the library user to specify where
raylib is stored.

src/build.zig

@@ -1,8 +1,6 @@
 const std = @import("std");
 
-pub fn Pkg(srcdir: []const u8) type {
+pub fn addRaylib(b: *std.build.Builder, target: std.zig.CrossTarget) *std.build.LibExeObjStep {
-    return struct {
-        pub fn addRaylib(b: *std.build.Builder, target: std.zig.CrossTarget) *std.build.LibExeObjStep {
     // Standard release options allow the person running `zig build` to select
     // between Debug, ReleaseSafe, ReleaseFast, and ReleaseSmall.
     const mode = b.standardReleaseOptions();
@@ -76,15 +74,9 @@ pub fn Pkg(srcdir: []const u8) type {
         },
     }
 
-            raylib.setOutputDir("./");
-            raylib.install();
     return raylib;
-        }
-    };
 }
 
-const lib = Pkg(".");
-
 pub fn build(b: *std.build.Builder) void {
     // Standard target options allows the person running `zig build` to choose
     // what target to build for. Here we do not override the defaults, which
@@ -92,5 +84,13 @@ pub fn build(b: *std.build.Builder) void {
     // for restricting supported target set are available.
     const target = b.standardTargetOptions(.{});
 
-    _ = lib.addRaylib(b, target);
+    const lib = addRaylib(b, target);
+    lib.setOutputDir(srcdir);
+    lib.install();
+}
+
+const srcdir = getSrcDir();
+
+fn getSrcDir() []const u8 {
+    return std.fs.path.dirname(@src().file) orelse ".";
 }