[Rd] --disable-long-double or --enable-long-double=no?

[Michael Chirico]

There's a bit of confusion about how to disable long double support in an R
build.

I see --disable-long-double scattered about, e.g.

   - R-exts:
   https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Writing-portable-packages
   - R-admin:
   https://cran.r-project.org/doc/manuals/r-release/R-admin.html#Solaris
   - CRAN noLD check description:
   https://www.stats.ox.ac.uk/pub/bdr/noLD/README.txt
   - ?capabilities:
   https://stat.ethz.ch/R-manual/R-devel/library/base/html/capabilities.html

However, it's *missing* from ./config (cd r-source && grep
"disable-long-double" configure). Instead there appears to be some code
built around enable-long-double:

./configure:1808:  --enable-long-double    use long double type [yes]

./configure:24723:# Check whether --enable-long-double was given.

I see the option apparently introduced here in 2012 & the ambiguity is
immediate -- the commit mentions disable-long-double but builds
enable-long-double.

https://github.com/wch/r-source/commit/fb8e36f8be0aaf47a9c54c9effb219dae34f0e41

Could someone please help to clear the confusion?

Thanks
Michael Chirico

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