cmake_policy
------------

Manage CMake Policy settings.

As CMake evolves it is sometimes necessary to change existing behavior
in order to fix bugs or improve implementations of existing features.
The CMake Policy mechanism is designed to help keep existing projects
building as new versions of CMake introduce changes in behavior.  Each
new policy (behavioral change) is given an identifier of the form
"CMP<NNNN>" where "<NNNN>" is an integer index.  Documentation
associated with each policy describes the OLD and NEW behavior and the
reason the policy was introduced.  Projects may set each policy to
select the desired behavior.  When CMake needs to know which behavior
to use it checks for a setting specified by the project.  If no
setting is available the OLD behavior is assumed and a warning is
produced requesting that the policy be set.

The cmake_policy command is used to set policies to OLD or NEW
behavior.  While setting policies individually is supported, we
encourage projects to set policies based on CMake versions.

::

  cmake_policy(VERSION major.minor[.patch[.tweak]])

Specify that the current CMake list file is written for the given
version of CMake.  All policies introduced in the specified version or
earlier will be set to use NEW behavior.  All policies introduced
after the specified version will be unset (unless variable
CMAKE_POLICY_DEFAULT_CMP<NNNN> sets a default).  This effectively
requests behavior preferred as of a given CMake version and tells
newer CMake versions to warn about their new policies.  The policy
version specified must be at least 2.4 or the command will report an
error.  In order to get compatibility features supporting versions
earlier than 2.4 see documentation of policy CMP0001.

::

  cmake_policy(SET CMP<NNNN> NEW)
  cmake_policy(SET CMP<NNNN> OLD)

Tell CMake to use the OLD or NEW behavior for a given policy.
Projects depending on the old behavior of a given policy may silence a
policy warning by setting the policy state to OLD.  Alternatively one
may fix the project to work with the new behavior and set the policy
state to NEW.

::

  cmake_policy(GET CMP<NNNN> <variable>)

Check whether a given policy is set to OLD or NEW behavior.  The
output variable value will be "OLD" or "NEW" if the policy is set, and
empty otherwise.

CMake keeps policy settings on a stack, so changes made by the
cmake_policy command affect only the top of the stack.  A new entry on
the policy stack is managed automatically for each subdirectory to
protect its parents and siblings.  CMake also manages a new entry for
scripts loaded by include() and find_package() commands except when
invoked with the NO_POLICY_SCOPE option (see also policy CMP0011).
The cmake_policy command provides an interface to manage custom
entries on the policy stack:

::

  cmake_policy(PUSH)
  cmake_policy(POP)

Each PUSH must have a matching POP to erase any changes.  This is
useful to make temporary changes to policy settings.

Functions and macros record policy settings when they are created and
use the pre-record policies when they are invoked.  If the function or
macro implementation sets policies, the changes automatically
propagate up through callers until they reach the closest nested
policy stack entry.