Help/command/OPTIONS_SHELL.txt - third_party/cmake - Git at Google

 The final set of compile or link options used for a target is constructed by
 accumulating options from the current target and the usage requirements of
 its dependencies.  The set of options is de-duplicated to avoid repetition.
 While beneficial for individual options, the de-duplication step can break
 up option groups.  For example, ``-D A -D B`` becomes ``-D A B``.  One may
 specify a group of options using shell-like quoting along with a ``SHELL:``
 prefix.  The ``SHELL:`` prefix is dropped, and the rest of the option string
 is parsed using the :command:`separate_arguments` ``UNIX_COMMAND`` mode.
 For example, ``"SHELL:-D A" "SHELL:-D B"`` becomes ``-D A -D B``.
	The final set of compile or link options used for a target is constructed by
	accumulating options from the current target and the usage requirements of
	its dependencies. The set of options is de-duplicated to avoid repetition.
	While beneficial for individual options, the de-duplication step can break
	up option groups. For example, ``-D A -D B`` becomes ``-D A B``. One may
	specify a group of options using shell-like quoting along with a ``SHELL:``
	prefix. The ``SHELL:`` prefix is dropped, and the rest of the option string
	is parsed using the :command:`separate_arguments` ``UNIX_COMMAND`` mode.
	For example, ``"SHELL:-D A" "SHELL:-D B"`` becomes ``-D A -D B``.