Fix and simplify make clean

--add-gpr-path was not supported by the clean command.
Remove the whole build directory instead of removing each project subdirectory.

Inspired by the github issue #90
created by David SAUVAGE - AdaLabs Ltd <[email protected]>.

Author: Jicquel

Diff for: Makefile

@@ -137,9 +137,7 @@ doc:
 ###########
 
 clean:
-	$(PYTHON) $(SOURCE_DIR)/projects/gnatcoll_projects.gpr.py clean --add-gpr-path=local-install/share/gpr
-	$(PYTHON) $(SOURCE_DIR)/core/gnatcoll_core.gpr.py clean --add-gpr-path=local-install/share/gpr
-	$(PYTHON) $(SOURCE_DIR)/minimal/gnatcoll_minimal.gpr.py clean --add-gpr-path=local-install/share/gpr
+	rm -rf local-install
 
 # Let gprbuild handle parallelisation. In general, we don't support parallel
 # runs in this Makefile, as concurrent gprinstall processes may crash.