science/py-pymatgen: fix build on GCC architectures

C++11 compiler is necessary:cc1plus:
error: unrecognized command line option "-std=c++11"

-march=native is specified in CXXFLAGS and may break runtime on some x86:
cc1plus: error: unrecognized command line option "-march=native"

Approved by: portmgr (ports compliance)