[Appveyor] Temporarily disable Gui build.

Add error message when trying to build Gui with CMake 3.9.x.

.appveyor.yml

@@ -43,8 +43,8 @@ before_build:
 
 build_script:
   - mkdir build & cd build
-  - conan install .. -o gui=True
-  - cmake .. -G "Ninja" -DCMAKE_BUILD_TYPE=%configuration% -DBUILD_SHARED_LIBS=OFF -DOGS_EIGEN_DYNAMIC_SHAPE_MATRICES=ON -DOGS_USE_PCH=OFF -DOGS_BUILD_GUI=ON
+  - conan install .. # -o gui=True
+  - cmake .. -G "Ninja" -DCMAKE_BUILD_TYPE=%configuration% -DBUILD_SHARED_LIBS=OFF -DOGS_EIGEN_DYNAMIC_SHAPE_MATRICES=ON -DOGS_USE_PCH=OFF # -DOGS_BUILD_GUI=ON
   - cmake --build . --config %configuration%
 
 test_script:

CMakeLists.txt

@@ -62,6 +62,11 @@ option(OGS_BUILD_CLI "Should the OGS simulator be built?" ON)
 option(OGS_BUILD_GUI "Should the Data Explorer be built?" OFF)
 if(OGS_BUILD_GUI)
     add_definitions(-DOGS_BUILD_GUI)
+    # TODO: Remove when this is fixed in CMake, maybe in 3.10
+    # https://gitlab.kitware.com/cmake/cmake/issues/17205
+    if(${CMAKE_VERSION} VERSION_GREATER_EQUAL 3.9.0)
+        message(FATAL_ERROR "OGS_BUILD_GUI is not supported with CMake 3.9.x. Please downgrade CMake to 3.8.x!")
+    endif()
 endif()
 option(OGS_BUILD_UTILS "Should the utilities programms be built?" OFF)