[Rd] complex tests failure

[Kasper Daniel Hansen]

For a while I have been getting that the complex tests fails on RHEL 6.
The specific issue has to do with tanh (see below for full output from
complex.Rout.fail).

This is both with the stock compiler (GCC 4.4.7) and a compiler supplied
through the conda project (GCC 4.8.5).  The compiler supplied through conda
ends up linking R to certain system files, so the binary is not completely
independent (although most dynamically linked libraries are coming from the
conda installation).

A search on R-devel reveals a discussion in April on an issue reported on
Windows with a bug in tanh in old versions of the GNU C standard library;
this seems relevant.  The discussion by Martin Maechler suggest "using R's
internal substitute".  So how do I enable this?  Or does this requires
updating the C standard library?

** From complex.Rout.fail

> stopifnot(identical(tanh(356+0i), 1+0i))
Error: identical(tanh(356 + (0+0i)), 1 + (0+0i)) is not TRUE
In addition: Warning message:
In tanh(356 + (0+0i)) : NaNs produced in function "tanh"
Execution halted

Best,
Kasper

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