[CMake] require v3.12 on Windows

This is to avoid having to write some more work-arounds

CMakeLists.txt

@@ -5,11 +5,11 @@
 # See STIR/LICENSE.txt for details
 
 # cmake file for building STIR. See the STIR User's Guide and http://www.cmake.org.
-cmake_minimum_required(VERSION 3.1.0)
-
-
-# avoid warning about WIN32 no longer defined in CYGWIN
-set(CMAKE_LEGACY_CYGWIN_WIN32 0) 
+if (CMAKE_HOST_WIN32)
+  cmake_minimum_required(VERSION 3.12.0)
+else()
+  cmake_minimum_required(VERSION 3.1.0)
+endif()
 
 # enable ccache https://ccache.samba.org/
 find_program(CCACHE_PROGRAM ccache)

documentation/release_5.2.htm

@@ -105,6 +105,7 @@ <h3>Deprecated functionality</h3>
 
 <h3>Build system and dependencies</h3>
 <ul>
+  <li>CMake 3.12 is now required on Windows. (It will be required on all platforms in the next version).</li>
   <li>We now use CMake's <a href=https://gitlab.kitware.com/cmake/community/-/wikis/doc/tutorials/Object-Library>OBJECT library feature</a> for the registries. This avoids re-compilation of the registries for every executable and therefore speeds-up building time. Use of STIR in an external project is not affected as long as the recommended practice was followed. This is now documented in the User's Guide.
     <br /><a href="https://github.com/UCL/STIR/pull/1141/">PR #1141</a>.
   </li>