math/py-rpy2: fix build on GCC architectures

Use C11 compiler:
rpy2/rinterface_lib/R_API_eventloop.h:24: error: redefinition of typedef 'InputHandler'
rpy2/rinterface_lib/R_API_eventloop.h:6: error: previous declaration of 'InputHandler' was here

Approved by: mentors (implicit approval)