Allow -DNO_STRICT_REGEX to restore historic regex behavior

Allow restoring the behavior of '{' as described in regex(3).
Ie. only treat it as start of bounds if followed by a digit.

If NO_STRICT_REGEX is not defined, the behavior introduced by
commit a4a801688c909ef39cbcbc3488bc4fdbabd69d66 is retained,
otherwise the previous behavior is restored.

Differential Revision: https://reviews.freebsd.org/D45134