Support POSIX regular expression support using PCRE in CMake builds.

This change is mainly meant for Windows regex support, where PCRE Windows development is much
more active than development of the regex library found in the GNUWin32 packages. I found that
the latest release of PCRE built right out of the box, unlike the GNUWin32 regex library released
6 years ago which failed to build from source for me using the mingw toolchain.