* 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.
Stephen Kelly