PreventInSourceBuilds.cmake: + header, for external use

cmake/commons/PreventInSourceBuilds.cmake

-# - Prevent in-source builds.
+# PreventInSourceBuilds.cmake
+#
+# Forbids in-source builds.
+#
+# Rationale:
+# There is no good reason to run CMake from the main source directory,
+# as this would mess up the directory with compilation output.
+# If CMake is nonetheless run in-source, then this is most likely by accident.
+# This script is meant to prevent this specific accident.
+#
+# Usage:
+# Insert the line "include(PreventInSourceBuilds)" near the top
+# of the top-level CMakeLists.txt.
+#
+# License:
+# No rights reserved (CC0).
+#
+# Author:
+# Scientific Computing Group of JCNS-MLZ.
+#
+# Copyright:
+# Forschungszentrum Jülich GmbH 2021
 
 function(prevent_in_source_builds)
   # make sure the user doesn't play dirty with symlinks