OpenWatcom: Allow specifying the runtime library

Add a `CMAKE_WATCOM_RUNTIME_LIBRARY` variable to control the
runtime library selection.  Add policy CMP0136 to switch to
in place of the old hard-coded default flags.

Fixes: #23178

Help/variable/CMAKE_WATCOM_RUNTIME_LIBRARY.rst

@@ -0,0 +1,36 @@
+CMAKE_WATCOM_RUNTIME_LIBRARY
+----------------------------
+
+.. versionadded:: 3.24
+
+Select the Watcom runtime library for use by compilers targeting the Watcom ABI.
+This variable is used to initialize the :prop_tgt:`WATCOM_RUNTIME_LIBRARY`
+property on all targets as they are created.  It is also propagated by
+calls to the :command:`try_compile` command into the test project.
+
+The allowed values are:
+
+.. include:: ../prop_tgt/WATCOM_RUNTIME_LIBRARY-VALUES.txt
+
+Use :manual:`generator expressions <cmake-generator-expressions(7)>` to
+support per-configuration specification.
+
+For example, the code:
+
+.. code-block:: cmake
+
+  set(CMAKE_WATCOM_RUNTIME_LIBRARY "MultiThreaded")
+
+selects for all following targets a multi-threaded statically-linked runtime
+library.
+
+If this variable is not set then the :prop_tgt:`WATCOM_RUNTIME_LIBRARY` target
+property will not be set automatically.  If that property is not set then
+CMake uses the default value ``MultiThreadedDLL`` to select a Watcom runtime
+library.
+
+.. note::
+
+  This variable has effect only when policy :policy:`CMP0136` is set to ``NEW``
+  prior to the first :command:`project` or :command:`enable_language` command
+  that enables a language using a compiler targeting the Watcom ABI.