maint.mk: use more readable (yet functionally equivalent) quoting

maint.mk: use more readable (yet functionally equivalent) quoting

It is common to quote a single quote in a single quoted string like
this:  '...'\''...'.  Unless you know the idiom, that looks like
gibberish, so prefer to double-quote the string when possible.
Then you can use a more readable, lone single quote: "...'..."
* top/maint.mk (sc_cast_of_argument_to_free): Quoting like this
"don't" is more readable than the equivalent 'don'\''t'.
(sc_cast_of_x_alloc_return_value): Likewise.
(sc_cast_of_alloca_return_value): Likewise.
(sc_makefile_path_separator_check): Similar: use ":" in '...',
rather than '\'':'\''.