Fix devel/rttr on GCC-based architectures.

• Use C++11-capable compiler
• Import fix from upstream's upstream,

https://github.com/Tencent/rapidjson/commit/fa5963a2f5b231ee2babff771f169ccca22870ed

No PORTREVISION bump since it didn't compile before, and the
patch doesn't change any functionality.

PR: 237697
Submitted by: pkubaj
Reported by: pkubaj