games/simutrans: unbreak with libc++ 5.0

In file included from squirrel/squirrel/sqvm.cc:5:
In file included from /usr/include/c++/v1/math.h:310:
/usr/include/c++/v1/limits:233:91: error: member reference base type 'int' is not a structure or union

_LIBCPP_INLINE_VISIBILITY static _LIBCPP_CONSTEXPR type quiet_NaN() _NOEXCEPT {return type(0);}
                                                                                      ^~~~~~~

squirrel/squirrel/sqobject.h:131:25: note: expanded from macro 'type'
#define type(obj) ((obj)._type)

~~~~~^~~~~~

PR: 222402
Submitted by: chardon.frederic@gmail.com
Reported by: pkg-fallout