Improve compatibility of compilation tests

The explicit `clean` is added for GNAT Community 2020 and Pro 20.2. The
compilation of the test suite with restrictions is disabled for FSF
GNAT, as this compiler compiles the AUnit source code, which is
incompatible to the restrictions, regardless of whether it is needed.

Ref. #911

Makefile

@@ -78,12 +78,14 @@ test_apps:
 	$(MAKE) -C examples/apps/dhcp_client test
 
 test_compilation:
-	$(MAKE) -C tests/spark build_strict
+	test "${GNAT}" = "fsf" || $(MAKE) -C tests/spark build_strict
+	$(MAKE) -C tests/spark clean
 	$(MAKE) -C tests/spark test
 	$(MAKE) -C examples/apps/ping build
 	$(MAKE) -C examples/apps/dhcp_client build
 	python3 -m pytest -n$(shell nproc) -vv -m "compilation and not verification" tests
 	$(MAKE) -C tests/spark test NOPREFIX=1
+	$(MAKE) -C tests/spark clean
 	$(MAKE) -C tests/spark test_optimized
 
 test_specs: