Enabling -Wundef option in GCC will warn about the following lines:
/home/arch/widelands/src/i18n.cc:31:5: warning: "__APPLE__" is not defined [-Wundef]
/home/arch/widelands/src/i18n.cc:34:7: warning: "_WIN32" is not defined [-Wundef]
/home/arch/widelands/src/wlapplication.cc:919:7: warning: "WIN32" is not defined [-Wundef]
/home/arch/widelands/src/wlapplication.cc:981:7: warning: "WIN32" is not defined [-Wundef]
/home/arch/widelands/src/wlapplication.cc:983:7: warning: "__APPLE__" is not defined [-Wundef]
/home/arch/widelands/src/wlapplication.cc:1624:5: warning: "WIN32" is not defined [-Wundef]
From checking a few, I guess it doesn't like that these lines use #if instead of #ifdef.
If we do decide to change these, we should probably coordinate it so that we can get people on the platforms in question to test whether things still work as intended afterwards.
Jepp you are likely right with #if vs #ifdef
I further want to note, that _WIN32 should be replaced by WIN32 :)