[cmake] Fix standalone configuration of examples project

[zaveshaa]

Problem

Configuring the examples as a standalone project currently fails:

cd examples && mkdir build && cd build && cmake ..
-- Configuring incomplete, errors occurred!
CMake Error at CMakeLists.txt:37 (add_if_flag_compiles):
  Unknown CMake command "add_if_flag_compiles".

Two causes:

1. add_if_flag_compiles() is defined in cmake/AddIfFlagCompiles.cmake, which is only included by the root project — never when examples are configured on their own
2. No cmake_minimum_required() in this file, which is a hard error on CMake 4.x

Fix

• Add cmake_minimum_required(VERSION 3.22), matching the root project
• Add ../cmake to CMAKE_MODULE_PATH and include AddIfFlagCompiles, so the pthread/c11 flag checks work standalone
• Default PLATFORM to Desktop when it isn't inherited from a parent raylib build

Verification (macOS 15, Intel, Apple clang 14)

• Before: cmake .. fails with Unknown CMake command "add_if_flag_compiles"
• After: standalone configure completes and generates build files for all examples
• Normal path unaffected: root project with -DBUILD_EXAMPLES=ON still configures and builds examples correctly

Note: after this fix, standalone configure succeeds but linking against an installed static raylib fails on macOS because the installed CMake package only propagates OpenGL.framework (Cocoa/IOKit are missing). That looks like a separate pre-existing issue — happy to file/fix it next if confirmed.