add_custom_command: Add CODEGEN support

By specifying CODEGEN as an argument to add_custom_command the
custom command will be added to a codegen build target.

The intent is to provide a convenient way for users to get
their generated files without having to build the whole project.

This can be helpful for code analysis tools which can be useful
for IDEs and CI.

Help/release/dev/codegen.rst

@@ -0,0 +1,10 @@
+codegen
+-------
+
+* The :ref:`Ninja Generators` and :ref:`Makefile Generators` now produce
+  a ``codegen`` build target.  See policy :policy:`CMP0171`.  It drives a
+  subset of the build graph sufficient to run custom commands created with
+  :command:`add_custom_command`'s new ``CODEGEN`` option.
+
+* The :command:`add_custom_command` command gained a ``CODEGEN`` option
+  to mark a custom commands outputs as dependencies of a ``codegen`` target.