Fix 'ISO C++17 does not allow dynamic exception spec' clang-8 error

Fix 'ISO C++17 does not allow dynamic exception spec' clang-8 error

Issue #287 (bdwgc).

Before this patch, clang 8 (and -std=c++1z) fails for gc_cpp.cc.
The error message produced is: ISO C++17 does not allow dynamic
exception specifications.

The "dynamic exception" syntax was declared deprecated in C++11 and
removed in C++17.

* gc_cpp.cc [!_MSC_VER && !__DMC__ && GC_NEW_DELETE_NEED_THROW
&& __cplusplus >= 201703L] (GC_DECL_NEW_THROW): Define to
noexcept(false); add comment.