Avoid "expression with side effects..." warning using ex_to<T>().

Avoid "expression with side effects..." warning using ex_to<T>().

Clang++ warns that instantiations of this inline template function
ex_to<T>(const ex &) are "expression with side effects [that] will be
evaluated despite being used as an operand to 'typeid'".

Let's declare this ex_to<T>() as a pure function (i.e. having no
side effects) for compilers which support __attribute__((__pure__)).

GCC seems to support this attribute since more than ten years.