warning: format string is not a string literal

[cpsievert]

CRAN has asked for a fix the following new problem

(from https://cran.r-project.org/web/checks/check_results_later.html)

Version: 1.3.1
Check: whether package can be installed
Result: WARN 
  Found the following significant warnings:
    later_posix.cpp:136:14: warning: format string is not a string literal (potentially insecure) [-Wformat-security]
  See ‘/home/hornik/tmp/R.check/r-devel-clang/Work/PKGS/later.Rcheck/00install.out’ for details.
  * used C compiler: ‘Debian clang version 17.0.5 (1)’
  * used C++ compiler: ‘Debian clang version 17.0.5 (1)’
Flavor: [r-devel-linux-x86_64-debian-clang](https://www.r-project.org/nosvn/R.check/r-devel-linux-x86_64-debian-clang/later-00check.html)