Fix #2621: Set CMP0054 policy to NEW and appropriately quote ${PLATFORM} (#2622)

CMakeLists.txt

@@ -1,6 +1,18 @@
 cmake_minimum_required(VERSION 3.0)
 project(raylib)
 
+# Avoid excessive expansion of variables in conditionals. In particular, if
+# "PLATFORM" is "DRM" than:
+#
+# if (${PLATFORM} MATCHES "DRM")
+#
+# may expand e.g to:
+#
+# if (/usr/lib/aarch64-linux-gnu/libdrm.so MATCHES "DRM")
+#
+# See https://cmake.org/cmake/help/latest/policy/CMP0054.html
+cmake_policy(SET CMP0054 NEW)
+
 # Directory for easier includes
 # Anywhere you see include(...) you can check <root>/cmake for that file
 set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake)