Since the PCRE2 library was always going to be a part of MaxScale, there was
no real reason to have it as a shared library apart from smaller binaries.
Switched to ExternalProject CMake module to make the inclusion of external CMake
projects easier. This removes the rebuilding of the pcre2 libraries each time
the project is reconfigured.
The bundled PCRE2 library will be built as a separate target and configuring
CMake no longer builds it. Instead, it will only be built when it is out of date.
This requires all targets to declare that they depend on the pcre2 target in
order for it to be built.
