Simplify demangling checks by Enchufa2 · Pull Request #1401 · RcppCore/Rcpp

Enchufa2

Closes #1400. As discussed there, technically __has_include was introduced in C++17, but in practice, being a preprocessor macro, compilers define it regardless of the standard set (tested locally with gcc and clang). Also, checking whether __has_include is defined first is safer as recommended by gcc.

Checklist

Code compiles correctly
R CMD check still passes all tests
Preferably, new tests were added which fail without the change
Document the changes by file in ChangeLog