Exceptions impl fixes

[ellert]

Some fixes for inst/include/Rcpp/exceptions_impl.h

• the header file should have an include guard like all the other headers.
• The RCPP_DEMANGLER_ENABLED should always have a value - missing #else
• demangler_one should have an inline qualifier. Without this ROOT's R interface complains:

Processing /builddir/build/BUILD/root-6.20.02/tutorials/r/Function.C...
IncrementalExecutor::executeFunction: symbol '_ZN4RcppL13demangler_oneEPKc' unresolved
while linking [cling interface function]!
You are probably missing the definition of Rcpp::demangler_one(char const*)
Maybe you need to load the corresponding shared library?

With the inline qualifier this error disappears.

Checklist

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

[kevinushey]

LGTM -- thanks for catching those.

[eddelbuettel]

Thumbs up, and thanks, from me too. Looks like we are breaking all sorts of (internal) record with that recent exceptions PR needing an unheard number of follow-ups 😭

Can you detail what makes ROOT's setup different so that it tripped here when this did not show anywhere else, including at CRAN?