Log Message:
textproc/py-rapidjson: Increase compiler requirements

On netbsd-9, some files failed to build with an ICE.  (The upstream
build system unhelpfully retried the build without the C extension,
making this harder to debug, but PLIST caught it.)

Declare C++17 because upstream README says so.
Require gcc 10, because gcc 7 on NetBSD 9 fails, and 10 is the new
normal.