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

cmake/FindSphinx.cmake

@@ -1,5 +1,5 @@
 # Ceres Solver - A fast non-linear least squares minimizer
-# Copyright 2015 Google Inc. All rights reserved.
+# Copyright 2023 Google Inc. All rights reserved.
 # http://ceres-solver.org/
 #
 # Redistribution and use in source and binary forms, with or without
@@ -37,27 +37,24 @@
 
 find_program(SPHINX_EXECUTABLE
              NAMES sphinx-build
-             PATHS
-               /usr/bin
-               /usr/local/bin
-               /opt/local/bin
+             PATHS /opt/local/bin
              DOC "Sphinx documentation generator")
 
 if (SPHINX_EXECUTABLE)
 
-  find_package(PythonInterp 3)
+  find_package(Python COMPONENTS Interpreter)
 
-  if(PYTHONINTERP_FOUND)
+  if(Python_Interpreter_FOUND)
     # Check for sphinx theme dependency for documentation
     execute_process(
-      COMMAND ${PYTHON_EXECUTABLE} -c "import sphinx_rtd_theme"
+      COMMAND ${Python_EXECUTABLE} -c "import sphinx_rtd_theme"
       RESULT_VARIABLE SPHINX_RTD_THEME
       OUTPUT_QUIET
       ERROR_QUIET
     )
   endif ()
 
-  if (${SPHINX_RTD_THEME} EQUAL 0)
+  if (SPHINX_RTD_THEME EQUAL 0)
     set(SPHINX_RTD_THEME TRUE)
   else ()
     set(SPHINX_RTD_THEME FALSE)