Simplify demangling checks

[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

[eddelbuettel]

That looks good to me