Fixed VTK dependency in build

CMakeLists.txt

@@ -1,3 +1,4 @@
+if (VTK_SUPPORT)
 include_directories(${CMAKE_INSTALL_PREFIX}/include)
 
 include_directories( "${CMAKE_CURRENT_SOURCE_DIR}/include" )
@@ -5,3 +6,6 @@ add_subdirectory("src")
 
 add_library( fem4ellipticpde ${FEMSOURCE} )  
 target_link_libraries(fem4ellipticpde "lemmacore")
+else()
+	message( STATUS "FEM4EllipticPDE requires Lemma to be build with VTK" )
+endif()