Compile with GNU c++

[hannes-ma]

Hello,
is there a way to compile this header library with GNU c++?
I get lots of errors using cpp 9.3.0, standard C++11, especially regarding constexpr overloading:

MatrixUtil/MatrixUtil/vec_types.h:109:20: error: ‘constexpr float& mutil::Vector2::operator[](size_t) const’ cannot be overloaded with ‘constexpr const float& mutil::Vector2::operator[](size_t) const’
  109 |   constexpr float &operator[](size_t index)
      |                    ^~~~~~~~
MatrixUtil/MatrixUtil/vec_types.h:104:26: note: previous declaration ‘constexpr const float& mutil::Vector2::operator[](size_t) const’
  104 |   constexpr const float &operator[](size_t index) const
      |                          ^~~~~~~~
MatrixUtil/MatrixUtil/vec_types.h: In member function ‘constexpr float& mutil::Vector2::operator[](size_t) const’:
MatrixUtil/MatrixUtil/vec_types.h:111:20: error: binding reference of type ‘float&’ to ‘const float’ discards qualifiers
  111 |    return vec[index];
      |           ~~~~~~~~~^
MatrixUtil/MatrixUtil/vec_types.h:112:3: error: body of ‘constexpr’ function ‘constexpr float& mutil::Vector2::operator[](size_t) const’ not a return-statement
  112 |   }
      |   ^

[evrhel]

I originally meant for this library to be used with C++ 14. This issue is on C++ 11 for me as well. I updated the README to reflect this.