Require at least CMake 3.16

Given we no longer support Ubuntu 18.04 due to packaged GCC lacking
C++17 support we can bump the minimum required CMake version to the one
provided by Ubuntu 20.04 which is CMake 3.16. Consequently, this allows
to drop some of the legacy CMake logic.

Change-Id: I1f05d4c5681d10aa7faa0800ef4a803be2f5b7dd

docs/source/CMakeLists.txt

@@ -9,12 +9,12 @@ install(DIRECTORY ${SPHINX_HTML_DIR}
         COMPONENT Doc
         PATTERN "${SPHINX_HTML_DIR}/*")
 
-# Find python 3
-find_package(PythonInterp 3 REQUIRED)
+# Find python
+find_package(Python REQUIRED COMPONENTS Interpreter)
 
 # Building using 'make_docs.py' python script
 add_custom_target(ceres_docs ALL
-                  ${PYTHON_EXECUTABLE}
+                  $<TARGET_FILE:Python::Interpreter>
                   "${Ceres_SOURCE_DIR}/scripts/make_docs.py"
                   "${Ceres_SOURCE_DIR}"
                   "${Ceres_BINARY_DIR}/docs"

docs/source/installation.rst

@@ -29,7 +29,7 @@ Ceres relies on a number of open source libraries, some of which are
 optional. For details on customizing the build process, see
 :ref:`section-customizing` .
 
-- `CMake <http://www.cmake.org>`_ 3.10 or later **required**.
+- `CMake <http://www.cmake.org>`_ 3.16 or later **required**.
 
 - `Eigen <http://eigen.tuxfamily.org/index.php?title=Main_Page>`_
   3.3 or later **required**.