Freeciv

#1334 changed tolua generated code to use nullptr instead of NULL, but its own toluabind.c source file was not regenerated with the updated tolua. Now, after toluabind.c does get regenerated, it will cause compilation error with older compilers, as building that source does not include freeciv's headers setting up fallback nullptr.

The two options I see are to either make our copy of tolua freeciv -specific by making it depend on freeciv headers. or to partially(?) revert #1334.