configure.ac: fix building against BusyBox sed
Welp, when you are not using -E
(extended) mode, basic POSIX regex
means that +
doesn't have to be escaped. Escaping it confuses some
version of sed, such as BusyBox sed in recent version of Alpine Linux
which fails with sed: bad regex '\+': Repetition not preceded by valid expression
.
POSIX regex, why do you have basic mode?
Fixes: #26 (closed)