CMake: reject in-tree builds.

We do not support clobbering the source tree with build products,
nor recommend this in any documentation, and this already leads to
weird bugs with the resource system. Show an explicit error in this
case to avoid confusion.

CMakeLists.txt

@@ -1,5 +1,12 @@
 # cmake configuration
 
+if(CMAKE_SOURCE_DIR STREQUAL CMAKE_BINARY_DIR)
+    message(FATAL_ERROR
+            "In-tree builds are not supported; please perform an out-of-tree build:\n"
+            "    rm -rf CMakeCache.txt CMakeFiles/\n"
+            "    mkdir build && cd build && cmake ..")
+endif()
+
 cmake_minimum_required(VERSION 3.1.0 FATAL_ERROR)
 cmake_policy(VERSION 3.1.0)
 set(CMAKE_MODULE_PATH ${CMAKE_MODULE_PATH}