Refine demangler setting and execinfo use

[eddelbuettel]

PR #1401 updated the detection for execinfo.h by relying on a header-finding macro directly which is powerful. It also removed a set of #if defined(...) tests precluding several architectures from trying. This now bit us in issue #1442 as on FreeBSD, the header is found ... but also requires a library.

Now, we could try to use autoconf to find header and library, and attempt to compile a test program (as we now do for OpenMP in RcppArmadillo and RcppEigen). But given that the affected architectures here are on the less-widely used side with difficult access for us to test and refine, a simpler strategy would be to punt and again rely on a set of #if define() so this PR restores this. That is plain KISS Principle: Keep it Simple (and) Stupid.

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

[kevinushey]

LGTM!

[bastistician]

FWIW and sorry to be a nuisance, I had suggested a KISS solution in #1400 (the "whitelist" approach, where I did mention that "BSDs have long provided execinfo, too, but a web search suggests they require separate linking with -lexecinfo"). That would be much simpler than the now-restored bunch of conditionals (including the non-standard __MUSL__), but I understand that this is now settled and should work everywhere, which is the main outcome of this revision. Thanks for maintaining portability!

[eddelbuettel]

Thanks. Might it be worthwhile to add the hint about linking needed to the short paragraph in R-exts.texi?

Header @file{execinfo.h} is only available on a few OSes: formerly nor
in MacOS nor Solaris, and currently not on Linux systems (such as Alpine
Linux) using @code{musl}.