lang/gdc: link error fixed
- GDC gets a fail of link with GCC42(on 8-current/7-PR1)
because libstdc++ is not in the default link target
where libphobos of GDC needs it.
To fix that issue, I have added a patch that make libstdc++
a link tareget of GDC.
Sometimes 6-stable says that it is a overplus link but it is
Submitted by: Masanori OZAWA <email@example.com> (maintainer)