CMake: define target_compile_features with C/C++ standards

[mwtoews]

This defines the high level meta features for C/C++ support based on their standard, and set using target_compile_features to the main proj target. It is PUBLIC, meaning that the C/C++ standard applies to compile this library, and anything that links to it. The exported target would look like this:

Properties for TARGET PROJ::proj:
  PROJ::proj.LOCATION = "/tmp/proj_shared_install_from_dist/lib/libproj.so.25.9.9.0"
  PROJ::proj.INTERFACE_INCLUDE_DIRECTORIES = "/tmp/proj_shared_install_from_dist/include"
  PROJ::proj.INTERFACE_LINK_LIBRARIES = <NOTFOUND>
  PROJ::proj.INTERFACE_COMPILE_FEATURES = "c_std_99;cxx_std_17"

Also tidy-up some of the messages while setting the CMake standard variables. These never fail, so message(CHECK_FAIL ...) is not needed (the failure, e.g. non-existing standard, happens before in project()).

Background information here.

Xref #1924

[kbevers]

@mwtoews do you want this included in 9.9.0?

[mwtoews]

@kbevers yes, and nothing else queued up.

[kbevers]

@kbevers yes, and nothing else queued up.

Perfect. It looks good to me so merging and including in release notes