Fix check to prevent in-tree builds (#2099)

* disable in-tree build

* check to prevent in-tree builds

.gitignore

@@ -111,3 +111,6 @@ python/share/*
 
 python/pybind11/
 
+# remnants from a failed in-source build
+CMakeCache.txt
+CMakeFiles/

CMakeLists.txt

@@ -16,6 +16,10 @@
 
 cmake_minimum_required (VERSION 3.1)
 
+if("${CMAKE_SOURCE_DIR}" STREQUAL "${CMAKE_BINARY_DIR}")
+    message(FATAL_ERROR "In-source builds are not allowed.")
+endif()
+
 include(cmake/Modules/git_tags.cmake)
 
 NGRAPH_GET_VERSION_LABEL()
@@ -74,10 +78,6 @@ if($ENV{NGRAPH_USE_PREBUILT_LLVM})
     set(NGRAPH_USE_PREBUILT_LLVM TRUE)
 endif()
 
-# These variables are undocumented but useful.
-set(CMAKE_DISABLE_SOURCE_CHANGES ON)
-set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)
-
 # Create compilation database compile_commands.json
 set(CMAKE_EXPORT_COMPILE_COMMANDS ON)