function
--------

Start recording a function for later invocation as a command::

  function(<name> [arg1 [arg2 [arg3 ...]]])
    COMMAND1(ARGS ...)
    COMMAND2(ARGS ...)
    ...
  endfunction(<name>)

Define a function named ``<name>`` that takes arguments named ``arg1``,
``arg2``, ``arg3``, (...).
Commands listed after function, but before the matching
:command:`endfunction()`, are not invoked until the function is invoked.
When it is invoked, the commands recorded in the function are first
modified by replacing formal parameters (``${arg1}``) with the arguments
passed, and then invoked as normal commands.
In addition to referencing the formal parameters you can reference the
``ARGC`` variable which will be set to the number of arguments passed
into the function as well as ``ARGV0``, ``ARGV1``, ``ARGV2``, ...  which
will have the actual values of the arguments passed in.
This facilitates creating functions with optional arguments.
Additionally ``ARGV`` holds the list of all arguments given to the
function and ``ARGN`` holds the list of arguments past the last expected
argument.
Referencing to ``ARGV#`` arguments beyond ``ARGC`` have undefined
behavior. Checking that ``ARGC`` is greater than ``#`` is the only way
to ensure that ``ARGV#`` was passed to the function as an extra
argument.

A function opens a new scope: see :command:`set(var PARENT_SCOPE)` for
details.

See the :command:`cmake_policy()` command documentation for the behavior
of policies inside functions.