Replace certain functions with GNULIB modules?

[hamogu]

The autoconf manual reports that some of the test macros marx currently uses are deprecated because GNULIB offers replacements fir those functions that do not require testing.
In particular, the autoconf macros AC_FUNC_STRTOD and AC_FUNC_ERROR_AT_LINE list possible GNULIB replacements in the autoconf manual.

I'm not sure it's worth the effort to include those since the required functions work on the systems that we support, but I'm opening this issue here to remind me to look into GNULIB for those functions, should we ever want to support systems where this becomes an issue.