Add new target-property `IMPORTED_GLOBAL`.

The purpose of this new `IMPORTED_GLOBAL` target-property is to prolong
the lifetime and scope of `IMPORTED` targets in such a way as if they
had been created with the keyword `GLOBAL` in the first place.

* It can only be set to `TRUE`. That means, a local `IMPORTED` target
  can be promoted to global scope but a global `IMPORTED` target cannot
  be degraded to local scope!
* Setting it to `TRUE` only succeeds if done from within the same
  directory in which the `IMPORTED` target was created in the first
  place.

Fixes #17256.

1 files changed, 22 insertions, 0 deletions

diff --git a/Help/prop_tgt/IMPORTED_GLOBAL.rst b/Help/prop_tgt/IMPORTED_GLOBAL.rst
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index 0000000..1feca04
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+++ b/Help/prop_tgt/IMPORTED_GLOBAL.rst
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+IMPORTED_GLOBAL
+---------------
+
+Indication of whether an :ref:`IMPORTED target <Imported Targets>` is
+globally visible.
+
+The boolean value of this property is True for targets created with the
+``IMPORTED`` ``GLOBAL`` options to :command:`add_executable()` or
+:command:`add_library()`. It is always False for targets built within the
+project.
+
+For targets created with the ``IMPORTED`` option to
+:command:`add_executable()` or :command:`add_library()` but without the
+additional option ``GLOBAL`` this is False, too. However, setting this
+property for such a locally ``IMPORTED`` target to True promotes that
+target to global scope. This promotion can only be done in the same
+directory where that ``IMPORTED`` target was created in the first place.
+
+Once an imported target has been made global, it cannot be changed back to
+non-global. Therefore, if a project sets this property, it may only
+provide a value of True. CMake will issue an error if the project tries to
+set the property to a non-True value, even if the value was already False.