cmake: disable fallthrough warnings for PCRE
Our PCRE dependency has uncommented fallthroughs in switch statements.
Turn off warnings for those in the PCRE code.
regexec: use pcre as our fallback/builtin regex
Use PCRE 8.42 as the builtin regex implementation, using its POSIX
compatibility layer. PCRE uses ASCII by default and the users locale
will not influence its behavior, so its `regcomp` implementation is
similar to `regcomp_l` with a C locale.