Compilation failure with c++11 (string_view)

[Jean-Marc Lasgouttes]

With master, I cannot compile in c++11 mode:

../../master/src/mathed/InsetMathDecoration.cpp:191:22: error: 
‘string_view’ has not been declared
   191 |   Attributes(bool o, string_view entity)
       |                      ^~~~~~~~~~~

Indeed, std::string_view is a c++17 thing.

JMarc