install: Implement new install(CODE|SCRIPT) option ALL_COMPONENTS

In a per-component installation the generated installation scripts
are invoked once for each component.

Per default custom installation script code added by install(CODE|SCRIPT)
only runs for one specific component in this context.

The new ALL_COMPONENTS option allows custom script code to be run once
for each component being installed.

Help/command/install.rst

@@ -619,7 +619,7 @@ Custom Installation Logic
 .. code-block:: cmake
 
   install([[SCRIPT <file>] [CODE <code>]]
-          [COMPONENT <component>] [EXCLUDE_FROM_ALL] [...])
+          [COMPONENT <component>] [EXCLUDE_FROM_ALL] [ALL_COMPONENTS] [...])
 
 The ``SCRIPT`` form will invoke the given CMake script files during
 installation.  If the script file name is a relative path it will be
@@ -634,6 +634,12 @@ example, the code
 
 will print a message during installation.
 
+The option ``ALL_COMPONENTS``
+  .. versionadded:: 3.21
+
+  Run the custom installation script code for every component of a
+  component-specific installation.
+
 .. versionadded:: 3.14
   ``<file>`` or ``<code>`` may use "generator expressions" with the syntax
   ``$<...>`` (in the case of ``<file>``, this refers to their use in the file

Help/manual/cmake-file-api.7.rst

@@ -751,6 +751,11 @@ with members:
     Optional member that is present with boolean value ``true`` when
     :command:`install` is called with the ``EXCLUDE_FROM_ALL`` option.
 
+  ``isForAllComponents``
+    Optional member that is present with boolean value ``true`` when
+    :command:`install(SCRIPT|CODE)` is called with the
+    ``ALL_COMPONENTS`` option.
+
   ``isOptional``
     Optional member that is present with boolean value ``true`` when
     :command:`install` is called with the ``OPTIONAL`` option.

Help/release/dev/install-script-all-components.rst

@@ -0,0 +1,7 @@
+install-script-all-components
+-----------------------------
+
+* The :command:`install(SCRIPT|CODE)` command
+  supports a new option ``ALL_COMPONENTS`` which allows
+  the corresponding code to run for every component of
+  a per component installation.