BundleUtilities: Disallow inclusion at configure time

This commit adds a new CMake policy, CMP0080, which prohibits the
inclusion of BundleUtilities at configure time. The old behavior is
to allow the inclusion.

Help/manual/cmake-policies.7.rst

@@ -57,6 +57,7 @@ Policies Introduced by CMake 3.13
 .. toctree::
    :maxdepth: 1
 
+   CMP0080: BundleUtilities cannot be included at configure time. </policy/CMP0080>
    CMP0079: target_link_libraries allows use with targets in other directories. </policy/CMP0079>
    CMP0078: UseSWIG generates standard target names. </policy/CMP0078>
    CMP0077: option() honors normal variables. </policy/CMP0077>

Help/policy/CMP0080.rst

@@ -0,0 +1,25 @@
+CMP0080
+-------
+
+:module:`BundleUtilities` cannot be included at configure time.
+
+The macros provided by :module:`BundleUtilities` are intended to be invoked
+at install time rather than at configure time, because they depend on the
+listed targets already existing at the time they are invoked. If they are
+invoked at configure time, the targets haven't been built yet, and the
+commands will fail.
+
+This policy restricts the inclusion of :module:`BundleUtilities` to
+``cmake -P`` style scripts and install rules. Specifically, it looks for the
+presence of :variable:`CMAKE_GENERATOR` and throws a fatal error if it exists.
+
+The ``OLD`` behavior of this policy is to allow :module:`BundleUtilities` to
+be included at configure time. The ``NEW`` behavior of this policy is to
+disallow such inclusion.
+
+This policy was introduced in CMake version 3.13.  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