cmTarget: Raise error if imported target location is not set

Previously we would synthesize <TARGET_NAME>-NOTFOUND as the location. This
would then end up on the link line and cause build failures.
Policy CMP0110 is added to control this behaviour.

Fixes #19080, #19943.

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.19
 .. toctree::
    :maxdepth: 1
 
+   CMP0111: An imported target with a missing location fails during generation. </policy/CMP0111>
    CMP0110: add_test() supports arbitrary characters in test names. </policy/CMP0110>
    CMP0109: find_program() requires permission to execute but not to read. </policy/CMP0109>
 

Help/policy/CMP0111.rst

@@ -0,0 +1,20 @@
+CMP0111
+-------
+
+.. versionadded:: 3.19
+
+An imported target with a missing location fails during generation.
+
+Prior to this the location would be generated as ``<TARGET_NAME>-NOTFOUND``,
+which would result in build failures.
+
+The ``OLD`` behavior of this policy is to generate the location of an imported
+unknown, static or shared library target as ``<TARGET_NAME>-NOTFOUND`` if not
+set.
+The ``NEW`` behavior is to raise an error.
+
+This policy was introduced in CMake version 3.19.  CMake version |release|
+warns when the policy is not set and uses ``OLD`` behavior. Use the
+:command:`cmake_policy` command to set it to ``OLD`` or ``NEW`` explicitly.
+
+.. include:: DEPRECATED.txt

Help/release/dev/imported-target-location-required.rst

@@ -0,0 +1,5 @@
+imported-target-location-required
+---------------------------------
+
+* An imported target with a missing location now fails during generation if the
+  location is used.  See policy :policy:`CMP0111`.