infra: pepper some GNU on top of Makefile

Makefile

@@ -14,6 +14,15 @@ CSC_FLAGS += -K prefix -static
 #     error.
 # -c: Stop after compilation to object files. This one is obvious.
 
+# GNU directory variables. Command line overrides, e.g.
+#   make prefix=$(HOME)/.local install
+#   make DESTDIR=/tmp/stage prefix=/usr install
+prefix = /usr/local
+exec_prefix = $(prefix)
+bindir = $(exec_prefix)/bin
+INSTALL = install
+INSTALL_PROGRAM = $(INSTALL)
+
 MODULE_FLAGS = -emit-all-import-libraries -module-registration -c
 
 # Order matters, since module check correctness on compilation
@@ -91,7 +100,7 @@ check-modules: sexc
 check-exit-code: sexc
 	@$(MAKE) --no-print-directory -C ./tests/exit-code check SEXC=../../sexc
 
-run-tests: sexc sex-tests sextest
+check run-tests: sexc sex-tests sextest
 	./sex-tests && ./sextest --sexc=./sexc $(SEX_TEST_PROGRAMS:%=./tests/sex-programs/%.sex) && $(MAKE) check-modules && $(MAKE) check-exit-code
 
 install: all installdirs
@@ -129,6 +138,13 @@ clean:
 	rm -f *.import.scm
 	rm -f *.link
 	rm -f sexc sex-tests sextest
+	$(MAKE) -C ./tests clean
 	$(MAKE) -C ./tests/modules clean
+	$(MAKE) -C ./tools/sextest clean
 
-.PHONY: clean depclean deps run-tests sex-tests sextest check-modules check-exit-code
+deps-clean:
+	rm -rf $(EGGS_DIR)
+
+.PHONY: all check run-tests check-modules check-exit-code \
+        install install-strip installdirs uninstall \
+        deps deps-update deps-clean clean sex-tests sextest