textproc/enchant: Fix build with ISPELL option
clang++ 16 defaults to C++17 causing errors with the codebase when the
ISPELL option is turned on. Switching back to C++11 (what upstream did
for 1.6.1) makes it build correctly.
PR: 273649
(cherry picked from commit ed633bad445cb49e524aab1cb11a2cbf17364a5c)