BUILD: Do not define LUA_USE_POSIX

It doesn't really do anything useful anymore anyway (except
make linux builds use _setjmp instead of setjmp, but both
are equally bad for us and need to replaced anyway).

configure

@@ -1634,7 +1634,6 @@ case $_host_os in
 		if test -z "$_host"; then
 			CXXFLAGS="$CXXFLAGS $(getconf LFS_CFLAGS 2>/dev/null)"
 		fi
-		DEFINES="$DEFINES -DLUA_USE_POSIX"
 		;;
 	mingw*)
 		DEFINES="$DEFINES -DWIN32"