Update math/gmm++ to 5.1

Due to it requiring c++11 features now, the consumers audio/csound
and audio/csound6 we're version bumped, and set USE_CXXSTD.

• http://download.gna.org/getfem/html/homepage/whatsnew/5.0.html
• http://download.gna.org/getfem/html/homepage/whatsnew/5.1.html

Reviewed by: rakuco
Approved by: rakuco (mentor)
Differential Revision: https://reviews.freebsd.org/D7730