OCT-5087 Use old cmake version again, see OCT-5322

CMakeLists.txt

@@ -11,8 +11,10 @@
 #    You should have received a copy of the FMILIB_License.txt file
 #    along with this program. If not, contact Modelon AB <http://www.modelon.com>.
 
-# CMake 3.1.3 or later is required to build Expat.
-cmake_minimum_required (VERSION 3.1.3 FATAL_ERROR)
+# CMake 3.1.3 or later is required to build Expat (as ExternalProject). However,
+# keeping 2.8.6 on the FMIL main project for now since updating causes issues
+# with CMP0026 (and more).
+cmake_minimum_required (VERSION 2.8.6 FATAL_ERROR)
 
 # Prohibit in-source build
 IF("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_BINARY_DIR}")