Separate clean from distclean

clean removes what the build produced and leaves the configured build
system, so a build following it proceeds without reconfiguring. Deleting
the build system from clean broke that: the target lives in the Makefile
it removes, so nothing in the directory could drive a rebuild afterward.

distclean empties the directory, build system included. It is terminal —
the next step is `cmake -B <dir>` from outside — and it is what proves a
change against a tree carrying no state from earlier runs.

CMakeLists.txt

@@ -132,23 +132,49 @@ add_subdirectory(docs ${CMAKE_BINARY_DIR}/docs-build)
 # ---------------------------------------------------------------------
 # Clean
 # ---------------------------------------------------------------------
-# clean removes what the build produced: the artifact directories, the
-# generated documentation, and the test scratch areas.
-#
-# The generated build system stays. CMakeCache.txt, the makefiles,
-# CMakeFiles/, and the CMake file API directory under .cmake/ are what an
-# IDE reads to know the project's targets exist; deleting them leaves the
-# directory unconfigured, so every target in the IDE stops resolving until
-# the project is reloaded. Removing the build system is what configuring a
-# fresh directory is for.
+# clean removes everything the build produced and leaves the configured
+# build system in place, so the next build proceeds from the directory
+# as it stands.
 set_property(DIRECTORY APPEND PROPERTY ADDITIONAL_CLEAN_FILES
-        ${DPM_TEST_RESULTS_DIR}
-        ${CMAKE_BINARY_DIR}/Testing
+        # Build artifacts.
         ${CMAKE_BINARY_DIR}/bin
         ${CMAKE_BINARY_DIR}/lib
         ${CMAKE_BINARY_DIR}/modules
         ${DPM_TEST_DIR}
         ${DPM_DOCS_DIR}
+
+        # Test scratch areas.
+        ${DPM_TEST_RESULTS_DIR}
+        ${CMAKE_BINARY_DIR}/Testing
+)
+
+# distclean empties the build directory, the generated build system
+# included, leaving what a freshly created directory holds: nothing.
+#
+# It is a terminal operation. Nothing inside the directory can drive a
+# build afterwards, so the next step is `cmake -B <dir>` from outside
+# it. Use it to prove a change against a tree carrying no state from
+# earlier runs.
+add_custom_target(distclean
+        COMMAND ${CMAKE_COMMAND} -E rm -rf
+                ${CMAKE_BINARY_DIR}/bin
+                ${CMAKE_BINARY_DIR}/lib
+                ${CMAKE_BINARY_DIR}/modules
+                ${DPM_TEST_DIR}
+                ${DPM_DOCS_DIR}
+                ${DPM_TEST_RESULTS_DIR}
+                ${CMAKE_BINARY_DIR}/Testing
+                ${CMAKE_BINARY_DIR}/CMakeFiles
+                ${CMAKE_BINARY_DIR}/CMakeCache.txt
+                ${CMAKE_BINARY_DIR}/Makefile
+                ${CMAKE_BINARY_DIR}/cmake_install.cmake
+                ${CMAKE_BINARY_DIR}/CTestTestfile.cmake
+                ${CMAKE_BINARY_DIR}/DartConfiguration.tcl
+                ${CMAKE_BINARY_DIR}/.cmake
+                ${CMAKE_BINARY_DIR}/CMakeDoxyfile.in
+                ${CMAKE_BINARY_DIR}/CMakeDoxygenDefaults.cmake
+                ${CMAKE_BINARY_DIR}/docs-build
+        COMMENT "Emptying the build directory"
 )
 
 # ---------------------------------------------------------------------