Set -Werror=return-local-addr on Unix build

Returning a reference to a temporary is such an obvious bug that I'm not sure
why compilers only treat it as a warning by default. This -W flag is tested on
Linux with G++, and might need changing for Clang.

CMakeLists.txt

@@ -7,6 +7,9 @@ project(darkradiant VERSION 2.11.0)
 set(CMAKE_CXX_STANDARD 17)
 set(CMAKE_CXX_STANDARD_REQUIRED True)
 
+# Global compiler options and warnings (TODO: may need updating for CLang)
+add_compile_options(-Werror=return-local-addr)
+
 # GCC 8 and earlier require explicit linking against stdc++fs
 if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9)
 	link_libraries(stdc++fs)