Add note that CUSTOMIZE_BUILD must be on if you want the options to apply.

CMake-Build-Options.md

@@ -1,4 +1,7 @@
 raylib is configurable and can be built in a variety of ways. Following is a listing of available CMake options and values, extracted from [`src/CMakeOptions.txt`](https://github.com/raysan5/raylib/blob/a1ec0a5bc33ab8726e55fa433ffc08fe3b42e539/src/CMakeOptions.txt).
+
+You must define `-DCUSTOMIZE_BUILD=ON` in order for many of these options to apply.
+
 > **TIP**:  You can use the curses UI provided by `ccmake(1)` for interactively configuring raylib.
 
 | **Option**                        | **Description**                                           | Possible Values (first is default)|