BUILD/MEDIUM: da Adding pcre2 support.

BUILD/MEDIUM: da Adding pcre2 support.

The DeviceAtlas Detection API now supports also the pcre2 library,
 and some users wish to have exclusively this version in their
environment.
Also, there is no longer new development happening in the legacy
 pcre(1) counterpart.
Simple check in the build process as the mutual exclusivity check between the
 two are already taking care of early on. Moving the check to the part
only when we build haproxy + the API from source as the other case the API is
 already built with the chosen regex library separately.