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cmake_minimum_required
----------------------
Set the minimum required version of cmake for a project and
update `Policy Settings`_ to match the version given::
cmake_minimum_required(VERSION <min>[...<max>] [FATAL_ERROR])
``<min>`` and the optional ``<max>`` are each CMake versions of the form
``major.minor[.patch[.tweak]]``, and the ``...`` is literal.
If the running version of CMake is lower than the ``<min>`` required
version it will stop processing the project and report an error.
The optional ``<max>`` version, if specified, must be at least the
``<min>`` version and affects policy settings as described below.
If the running version of CMake is older than 3.12, the extra ``...``
dots will be seen as version component separators, resulting in the
``...<max>`` part being ignored and preserving the pre-3.12 behavior
of basing policies on ``<min>``.
The ``FATAL_ERROR`` option is accepted but ignored by CMake 2.6 and
higher. It should be specified so CMake versions 2.4 and lower fail
with an error instead of just a warning.
.. note::
Call the ``cmake_minimum_required()`` command at the beginning of
the top-level ``CMakeLists.txt`` file even before calling the
:command:`project` command. It is important to establish version
and policy settings before invoking other commands whose behavior
they may affect. See also policy :policy:`CMP0000`.
Calling ``cmake_minimum_required()`` inside a :command:`function`
limits some effects to the function scope when invoked. Such calls
should not be made with the intention of having global effects.
Policy Settings
^^^^^^^^^^^^^^^
The ``cmake_minimum_required(VERSION)`` command implicitly invokes the
:command:`cmake_policy(VERSION)` command to specify that the current
project code is written for the given range of CMake versions.
All policies known to the running version of CMake and introduced
in the ``<min>`` (or ``<max>``, if specified) version or earlier will
be set to use ``NEW`` behavior. All policies introduced in later
versions will be unset. This effectively requests behavior preferred
as of a given CMake version and tells newer CMake versions to warn
about their new policies.
When a ``<min>`` version higher than 2.4 is specified the command
implicitly invokes::
cmake_policy(VERSION <min>[...<max>])
which sets CMake policies based on the range of versions specified.
When a ``<min>`` version 2.4 or lower is given the command implicitly
invokes::
cmake_policy(VERSION 2.4[...<max>])
which enables compatibility features for CMake 2.4 and lower.
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