Add --enable-enchant configure flag

In order to use the pkg-config macros, aclocal has to be run before autoconf.
This copies pkg.m4 to aclocal.m4 in src/ so it's available for autoconf, which
means it needs to be removed in the autoconf target.
Makefile: Add ENCHANT_CFLAGS/LIBS
Enchant requires +multi_byte
spell.c: Initial support for loading a dictionary from Enchant

A few Enchant-specific items have been added to slang_S.

The Enchant broker is initialized/freed appropriately.

If a spellfile isn't found and SpellFileMissing fails, fall back to loading a
dictionary from Enchant.  slang_free handles freeing the dictionary.
Add error-checking around convert_setup calls
Add suggest_try_enchant and use it from spell_suggest_intern

spell_check needs to be updated to validate the suggestions from Enchant,
otherwise they all get discarded as bogus suggestions.
Actually spell-check for Enchant-backed languages in spell_check
Support flagging words as good/bad with Enchant