Previously spell checker ignored text in single quotes however this
meant incorrect spelling was ignored in text where it shouldn't have
been.
In cases single quotes were used for literal strings
(such as variables, code & compiler flags),
replace these with back-ticks.
In cases they were used for UI labels,
replace these with double quotes.
In cases they were used to reference symbols,
replace them with doxygens symbol link syntax (leading hash).
Apply some spelling corrections & tweaks (for check_spelling_* targets).
c90e8bae0b