Code review comment for lp:~hjd/widelands/compilation-warnings

A good idea i think. I merged this and have not seen any problems with building widelands so far. Testing for individual warnings will likely need a script to probe the compiler for what it supports.

-Wno-attributes was added by me IIRC because compilation on old gcc version made some noisy output because we use attribute hot in some places. My memory might fail me though, I am an old man after all.