LINK_OPTIONS: Add support of "LINKER:" prefix

Help/variable/CMAKE_LANG_LINKER_WRAPPER_FLAG.rst

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+CMAKE_<LANG>_LINKER_WRAPPER_FLAG
+--------------------------------
+
+Defines the syntax of compiler driver option to pass options to the linker
+tool. It will be used to translate the ``LINKER:`` prefix in the link options
+(see :command:`add_link_options` and :command:`target_link_options`).
+
+This variable holds a :ref:`;-list <CMake Language Lists>` of tokens.
+If a space (i.e. " ") is specified as last token, flag and ``LINKER:``
+arguments will be specified as separate arguments to the compiler driver.
+The :variable:`CMAKE_<LANG>_LINKER_WRAPPER_FLAG_SEP` variable can be specified
+to manage concatenation of arguments.
+
+For example, for ``Clang`` we have:
+
+.. code-block:: cmake
+
+  set (CMAKE_C_LINKER_WRAPPER_FLAG "-Xlinker" " ")
+
+Specifying ``"LINKER:-z defs"`` will be transformed in
+``-Xlinker -z -Xlinker defs``.
+
+For ``GNU GCC``:
+
+.. code-block:: cmake
+
+  set (CMAKE_C_LINKER_WRAPPER_FLAG "-Wl,")
+  set (CMAKE_C_LINKER_WRAPPER_FLAG_SEP ",")
+
+Specifying ``"LINKER:-z defs"`` will be transformed in ``-Wl,-z,defs``.
+
+And for ``SunPro``:
+
+.. code-block:: cmake
+
+  set (CMAKE_C_LINKER_WRAPPER_FLAG "-Qoption" "ld" " ")
+  set (CMAKE_C_LINKER_WRAPPER_FLAG_SEP ",")
+
+Specifying ``"LINKER:-z defs"`` will be transformed in ``-Qoption ld -z,defs``.

Help/variable/CMAKE_LANG_LINKER_WRAPPER_FLAG_SEP.rst

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+CMAKE_<LANG>_LINKER_WRAPPER_FLAG_SEP
+------------------------------------
+
+This variable is used with :variable:`CMAKE_<LANG>_LINKER_WRAPPER_FLAG`
+variable to format ``LINKER:`` prefix in the link options
+(see :command:`add_link_options` and :command:`target_link_options`).
+
+When specified, arguments of the ``LINKER:`` prefix will be concatenated using
+this value as separator.