Enable building as a link in dunecontrol chain

Dunecontrol scans the module directory for certain files and phrases
which must be present if the project can be used as a dependency for
other DUNE modules.

cmake/Scripts/DuneCompat2.cmake

@@ -0,0 +1,18 @@
+# - Emulate a rule to patch the Makefile, adding a line to the source
+# tree and write a marker file indicating it is done.
+
+set (base_dir ".")
+set (marker_file "${base_dir}/CMakeFiles/marker")
+set (makefile "${base_dir}/Makefile")
+
+# if the Makefile has changed, then update it and touch the marker so that
+# the marker is newer and won't update the Makefile again
+if ("${makefile}" IS_NEWER_THAN "${marker_file}")
+  file (APPEND "${makefile}"
+	"abs_top_srcdir = ${CMAKE_HOME_DIRECTORY}\n"
+	)
+  execute_process (COMMAND
+	${CMAKE_COMMAND} -E touch "${marker_file}"
+	)
+endif ("${makefile}" IS_NEWER_THAN "${marker_file}")
+