[CMake] Check for CMake 3.4 when using Conan.

Fixes #1680.

scripts/cmake/ConanSetup.cmake

@@ -14,6 +14,7 @@ elseif(EXISTS ${CMAKE_SOURCE_DIR}/conanbuildinfo.cmake)
     set(USE_CONAN true)
 endif()
 if(USE_CONAN)
+    cmake_minimum_required(VERSION 3.4) # Conan Boost package requires this
     set(CMAKE_MODULE_PATH ${CONAN_CMAKE_MODULE_PATH} ${CMAKE_MODULE_PATH})
     message(STATUS "Using conan package manager")
     set(OGS_LIB_VTK System CACHE INTERNAL "")