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.

CMakeLists.txt

@@ -446,3 +446,6 @@ file (COPY
 	GNUmakefile
 	DESTINATION ${PROJECT_BINARY_DIR}
 	)
+
+# provide compatibility with using this build in dunecontrol
+include (DuneCompat)

cmake/Modules/DuneCompat.cmake

@@ -0,0 +1,23 @@
+# - Dunecontrol compatibility
+#
+# Enables this build to be a part of a dunecontrol chain. The
+# DUNE_CHECK_MODULES macro greps the top-level Makefile for the
+# presence of the abs_top_srcdir variable (!) and uses that as
+# the include directory of a module. Also, the contents are not
+# checked so if the variable is not present, it generates an
+# invalid command line (-I without argument) which causes the
+# autoconf probe to fail. This module patches our Makefile (!!)
+# so the necessary string will be there; in itself this string
+# has no use for us, it is solemnly to satisfy the M4 scripts.
+
+if (CMAKE_GENERATOR MATCHES "Unix Makefiles")
+  # we need an up-to-date, patched Makefile. this is always checked when
+  # the makefile is run, thus the need for a marker file to keep a
+  # timestamp to see when it was last patched (by us)
+  # amazingly, nothing depends on the generated Makefile, so this can be
+  # run whenever in the build without trigging a compile of e.g. config.h
+  add_custom_target (dune-compat ALL
+	COMMAND ${CMAKE_COMMAND} -DCMAKE_HOME_DIRECTORY=${CMAKE_HOME_DIRECTORY} -P ${PROJECT_SOURCE_DIR}/cmake/Scripts/DuneCompat2.cmake
+	COMMENT "Patching Makefile to be DUNE compatible"
+	)
+endif (CMAKE_GENERATOR MATCHES "Unix Makefiles")

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}")
+