Ensure that we always use lowercase function names, and that we do not
have spaces preceding open parentheses, for consistency.

We included their COPYING file, which was _not_ in fact their license.
Add the LICENSE file as well.

Our PCRE dependency has uncommented fallthroughs in switch statements.
Turn off warnings for those in the PCRE code.

PCRE includes compatibility functions that may go unused.  Don't warn.

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.