fail to install with gcc trunk which is due to be released as gcc
10.0.x early in 2020. It defaults to -fno-common, which stricter on
some multiply-defined symbols -- see §1.6.4.1 of 'Writing R Extensions'
in current R-devel.
We expect the issues to also be seen when compiling with earlier
versions of gcc using the (non-default) flag -fno-common: this has been
confirmed for gcc 9 on Linux, and also for clang on macOS.
Please correct as soon as possible and before Jan 10 to safely retain on
CRAN. (This is after the CRAN shutdown from Dec 20 to Jan 6.)
from cran maintainers email: