CPack: Use project version as default for `CPACK_PACKAGE_VERSION`

* Introduce `CMAKE_PROJECT_VERSION` and the corresponsing components:
  `CMAKE_PROJECT_VERSION_MAJOR`, `CMAKE_PROJECT_VERSION_MINOR`,
  `CMAKE_PROJECT_VERSION_PATCH` and `CMAKE_PROJECT_VERSION_TWEAK`.

* `CPack` module use `CMAKE_PROJECT_VERSION_MAJOR`,
  `CMAKE_PROJECT_VERSION_MINOR` and `CMAKE_PROJECT_VERSION_PATCH`
  to initialize corresponsing CPack variables.

1 files changed, 35 insertions, 0 deletions

diff --git a/Help/variable/CMAKE_PROJECT_VERSION.rst b/Help/variable/CMAKE_PROJECT_VERSION.rst
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+++ b/Help/variable/CMAKE_PROJECT_VERSION.rst
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+CMAKE_PROJECT_VERSION
+---------------------
+
+The version of the top level project.
+
+This variable holds the version of the project as specified in the top
+level CMakeLists.txt file by a :command:`project` command.  In the event that
+the top level CMakeLists.txt contains multiple :command:`project` calls,
+the most recently called one from that top level CMakeLists.txt will determine
+the value that ``CMAKE_PROJECT_VERSION`` contains.  For example, consider
+the following top level CMakeLists.txt:
+
+.. code-block:: cmake
+
+  cmake_minimum_required(VERSION 3.0)
+  project(First VERSION 1.2.3)
+  project(Second VERSION 3.4.5)
+  add_subdirectory(sub)
+  project(Third VERSION 6.7.8)
+
+And ``sub/CMakeLists.txt`` with the following contents:
+
+.. code-block:: cmake
+
+  project(SubProj VERSION 1)
+  message("CMAKE_PROJECT_VERSION = ${CMAKE_PROJECT_VERSION}")
+
+The most recently seen :command:`project` command from the top level
+CMakeLists.txt would be ``project(Second ...)``, so this will print::
+
+  CMAKE_PROJECT_VERSION = 3.4.5
+
+To obtain the version from the most recent call to :command:`project` in
+the current directory scope or above, see the :variable:`PROJECT_VERSION`
+variable.