In UTF-8 locales, the ranges A-F and a-f include several accented
characters beyond "ABCDEF" and "abcdef", such as the Croat č and ć.
This fixes https://savannah.gnu.org/bugs/?61487.
Bug existed since the beginning of each of the syntaxes,
for the C syntax since version 1.3.6, commit 159bdfdc.