Do not change minimum required version in modules

Some modules change CMake minimum required version when they are
included. For example:

  cmake_minimum_required(VERSION 2.8.12)
  message("${CMAKE_MINIMUM_REQUIRED_VERSION}")
  include(CheckTypeSize)
  message("${CMAKE_MINIMUM_REQUIRED_VERSION}")

will produce the following output:

  2.8.12
  2.6

This patch ensures that when you include a CMake module the minimum
required version and the policies set are left unchanged.

Fixes Issue #14864

Modules/CheckTypeSize.cmake

@@ -79,7 +79,7 @@ include(CheckIncludeFile)
 include(CheckIncludeFileCXX)
 
 cmake_policy(PUSH)
-cmake_minimum_required(VERSION 2.6 FATAL_ERROR)
+cmake_policy(VERSION 3.0)
 
 get_filename_component(__check_type_size_dir "${CMAKE_CURRENT_LIST_FILE}" PATH)
 

Modules/FindCUDA.cmake

@@ -330,11 +330,6 @@
 
 # FindCUDA.cmake
 
-# We need to have at least this version to support the VERSION_LESS argument to 'if' (2.6.2) and unset (2.6.3)
-cmake_policy(PUSH)
-cmake_minimum_required(VERSION 2.6.3)
-cmake_policy(POP)
-
 # This macro helps us find the location of helper files we will need the full path to
 macro(CUDA_FIND_HELPER_FILE _name _extension)
   set(_full_name "${_name}.${_extension}")

Modules/FindGTK2.cmake

@@ -572,7 +572,6 @@ endif()
 # If specified, enforce version number
 #
 if(GTK2_FIND_VERSION)
-    cmake_minimum_required(VERSION 2.6.2)
     set(GTK2_FAILED_VERSION_CHECK true)
     if(GTK2_DEBUG)
         message(STATUS "[FindGTK2.cmake:${CMAKE_CURRENT_LIST_LINE}] "