Backports support for extern(C++, "namespace"), which makes the
core.stdcpp package compilable.

Added predefined condition for CppRuntime_Gcc unconditionally, as it is
unlikely that D code will be linking to anything other than libstdc++
when extern(C++) is used.

Reviewed-on: https://github.com/dlang/dmd/pull/9371

gcc/d/ChangeLog:

2019-03-01  Iain Buclaw  <ibuclaw@gdcproject.org>

	* d-builtins.cc (d_init_versions): Add CppRuntime_Gcc as predefined
	version condition.

From-SVN: r269304