[CMAKE] Adding allias for pqxx target

[alexv-ds]

Why?

This will improve consistency between different ways of adding libpqxx to the project (specifically between find_package(libpqxx) and add_subdirectory(/subproject/path))

Example code that will be perfectly clean after this change

There is a minimalistic batch manager for cmake that is a thin wrapper over FetchContent, with some improvements - CPM.cmake. It has the ability to check for an installed version via find_package() before loading a package and the lack of consistency between find_package() and add_subriddectory() breaks the beauty of the code.

CPMAddPackage(
  NAME libpqxx
  GIT_TAG 7.8.1
  GITHUB_REPOSITORY jtv/libpqxx
)

target_link_libraries(${PROJECT_NAME} PRIVATE libpqxx::pqxx) # fail with add_subdirectory(), success with find_package()
target_link_libraries(${PROJECT_NAME} PRIVATE pqxx) # fail with find_package(), success with add_subdirectory()

# Note - in the second case the error will not be at configuration time, but at linking time.

If we add an alias to pqxx - the problem is solved. We will be able to use libpqxx::pqxx in both cases.

[jtv]

Thank you! @alexv-ds I did see a note from @tt4g in my email, but I don't see it here. I guess that has already been addressed?

[jtv]

Thank you! @alexv-ds I did see a note from @tt4g in my email, but I don't see it here. I guess that has already been addressed?

Oh never mind, I think that was an entirely different PR that's already been retracted!