* 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.
This means that we can use expressions of the form
$<TARGET_PROPERTY:foo,INTERFACE_INCLUDE_DIRECTORIES>
to get a list of the interface include directories of foo, including
those coming from dependencies.
We can't have a test of a target which has a single include directory in
its INCLUDE_DIRECTORIES because the shell on the MSYS platforms transforms
a single include directory to include a prefix, which is not what the test
expects. We test a target with two directories instead as a means to
test a target with no link dependencies.
