Add the ALIAS target concept for libraries and executables.
* The ALIAS name must match a validity regex. * Executables and libraries may be aliased. * An ALIAS acts immutable. It can not be used as the lhs of target_link_libraries or other commands. * An ALIAS can be used with add_custom_command, add_custom_target, and add_test in the same way regular targets can. * The target of an ALIAS can be retrieved with the ALIASED_TARGET target property. * An ALIAS does not appear in the generated buildsystem. It is kept separate from cmMakefile::Targets for that reason. * A target may have multiple aliases. * An ALIAS target may not itself have an alias. * An IMPORTED target may not have an alias. * An ALIAS may not be exported or imported.