diff --git a/cmake/PreventInSourceBuilds.cmake b/cmake/PreventInSourceBuilds.cmake
index be0794e4d..6488cedd9 100644
--- a/cmake/PreventInSourceBuilds.cmake
+++ b/cmake/PreventInSourceBuilds.cmake
@@ -7,13 +7,25 @@
 # in a dedicated build directory.  To prevent users from accidentally running
 # CMake in the source directory, just include this module.
 
-# make sure the user doesn't play dirty with symlinks
-get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
-get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+option(ALLOW_IN_SOURCE_BUILD "[deprecated] Allow building in source tree" OFF)
 
-# disallow in-source builds
-if("${srcdir}" STREQUAL "${bindir}")
-     message(FATAL_ERROR "\
+if (ALLOW_IN_SOURCE_BUILD)
+	set(CMAKE_DISABLE_SOURCE_CHANGES OFF CACHE INTERNAL "policy")
+	set(CMAKE_DISABLE_IN_SOURCE_BUILD OFF CACHE INTERNAL "policy")
+	if("${srcdir}" STREQUAL "${bindir}")
+		message(WARNING "Running in-source-tree build [ALLOW_IN_SOURCE_BUILD=ON]")
+	endif()
+else()
+	set(CMAKE_DISABLE_SOURCE_CHANGES ON CACHE INTERNAL "policy")
+	set(CMAKE_DISABLE_IN_SOURCE_BUILD ON CACHE INTERNAL "policy")
+
+	# make sure the user doesn't play dirty with symlinks
+	get_filename_component(srcdir "${CMAKE_SOURCE_DIR}" REALPATH)
+	get_filename_component(bindir "${CMAKE_BINARY_DIR}" REALPATH)
+
+	# disallow in-source builds
+	if("${srcdir}" STREQUAL "${bindir}")
+			 message(FATAL_ERROR "\
 
 CMake must not to be run in the source directory. \
 Rather create a dedicated build directory and run CMake there. \
@@ -21,12 +33,14 @@ CMake now already created some files, to clean up after this aborted in-source c
    rm -r CMakeCache.txt CMakeFiles
 or
    git clean -xdf
+
+If you happen to require in-source-tree builds for some reason rerun with -DALLOW_IN_SOURCE_BUILD=ON
 ")
-endif()
+	endif()
 
 # Check for remnants of in source builds
-if(EXISTS "${CMAKE_SOURCE_DIR}/CMakeCache.txt" OR EXISTS "${CMAKE_SOURCE_DIR}/CMakeFiles")
-	message(FATAL_ERROR " \
+	if(EXISTS "${CMAKE_SOURCE_DIR}/CMakeCache.txt" OR EXISTS "${CMAKE_SOURCE_DIR}/CMakeFiles")
+		message(FATAL_ERROR " \
 
 Remnants of in source CMake run detected, aborting!
 
@@ -34,5 +48,8 @@ To clean up after this aborted in-source compilation:
    rm -r CMakeCache.txt CMakeFiles
 or
    git clean -xdf
+
+If you happen to require in-source-tree builds for some reason rerun with -DALLOW_IN_SOURCE_BUILD=ON
 ")
+	endif()
 endif()