Use standard CMake constructs to export the library's targets. (#260)

* Use standard CMake constructs to export the library's targets.

* Fix typo.

* Document that PCRE2_USE_STATIC_LIBS is not required if only one edition of the library has been built.

* Replace some missed stuff from previous review.

* Don't explicitly set `PCRE2_FOUND` to true.

* Avoid defining the target aliases if they are already defined.

* Set `PCRE2_LIBRARIES` and `PCRE2_***_LIBRARY` for compatibility.

* Change `PCRE2_USE_STATIC_LIBS` to prohibit loading the shared library if set.