math/poly2tri: fix build on GCC architectures

Use C11 compiler:
../poly2tri/common/shapes.cc: In function 'bool p2t::IsDelaunay(const std::vector<p2t::Triangle*, std::allocator<p2t::Triangle*> >&)':
../poly2tri/common/shapes.cc:394: error: a function-definition is not allowed here before ':' token
../poly2tri/common/shapes.cc:409: error: expected primary-expression at end of input