CMake: Update for 3.1 compatibility

CMake 3.1's if() by default now interprets quoted variable names as
strings. This is a good idea from a strictness perspective, so make it
do this even if we're in 2.8.9 compatibility mode.

CMakeLists.txt

@@ -13,6 +13,13 @@
 
 cmake_minimum_required (VERSION 2.8.9)
 project (openclonk CXX C)
+
+# CMP0054: Only interpret if() arguments as variables or keywords when unquoted
+# (since CMake 3.1)
+if(POLICY CMP0054)
+	cmake_policy(SET CMP0054 NEW)
+endif()
+
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
 include(Version.txt)

cmake/CheckIncludeFilesCXX.cmake

@@ -45,7 +45,7 @@
 #=============================================================================
 
 macro(CHECK_INCLUDE_FILES_CXX INCLUDE VARIABLE)
-  if("${VARIABLE}" MATCHES "^${VARIABLE}$")
+  if(NOT DEFINED "${VARIABLE}")
     set(CMAKE_CONFIGURABLE_FILE_CONTENT "/* */\n")
     if(CMAKE_REQUIRED_INCLUDES)
       set(CHECK_INCLUDE_FILES_INCLUDE_DIRS "-DINCLUDE_DIRECTORIES=${CMAKE_REQUIRED_INCLUDES}")