math/gringo: Require a C++11-compliant standard library and make it work with GCC < 4.9.

[rakuco]

Summary

Despite ports r340913, the port still fails to build on FreeBSD 9.3 with lang/gcc (GCC 4.8):

libreify/src/parser.cc: In member function 'void Reify::Parser::parseProgram()':
libreify/src/parser.cc:130:51: error: 'to_string' is not a member of 'std'

Apply a workaround for bug 193528 found in other ports to make that function visible to GCC 4.8 (GCC 4.9 and later are fine).

Additionally, the port actually requires a C++11 standard library in addition to a C++11-compliant compiler, so adjust the USES line and later simplify the process of choosing which compiler to use.

[vsevolod]

Thank you, I've searched for something like this for a long time already!