CMake: Add a global disable flag for the CPU backend

The CPU backend is disabled by default and must be enabled with
the CMake variable NGRAPH_CPU_ENABLE set to true

cmake/external_llvm.cmake

@@ -13,7 +13,7 @@
 
 include(ExternalProject)
 
-if((NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin") AND
+if(NGRAPH_CPU_ENABLE AND (NOT ${CMAKE_SYSTEM_NAME} MATCHES "Darwin") AND
                          (NOT ${CMAKE_SYSTEM_NAME} MATCHES "Windows"))
     message(STATUS "Fetching LLVM from llvm.org")
     set(LLVM_RELEASE_URL http://releases.llvm.org/5.0.0/clang+llvm-5.0.0-linux-x86_64-ubuntu16.04.tar.xz)