Rise minimum required CMake version for Linux to 3.2

This will allow us to use modules like FindIntl.
https://lists.gnucash.org/logs/2019/05/14.html#T17:32:19

CMakeLists.txt

@@ -3,7 +3,7 @@
 if (WIN32 OR APPLE)
   cmake_minimum_required (VERSION 3.3.2)
 else()
-  cmake_minimum_required (VERSION 3.0)
+  cmake_minimum_required (VERSION 3.2)
 endif()
 
 project (gnucash)