devel/simgear: unbreak with libc++ 8

In file included from simgear/bucket/newbucket.cxx:30:
In file included from /usr/include/c++/v1/cmath:305:
In file included from /usr/include/c++/v1/math.h:309:
In file included from /usr/include/c++/v1/type_traits:406:
In file included from /usr/include/c++/v1/cstddef:38:
version:1:1: error: expected unqualified-id
2018.3.2
^

PR: 236192
Approved by: portmgr blanket