build-sys: add utility target to build TAGS

Author: keszybz

.gitignore

@@ -1,6 +1,7 @@
 __pycache__/
 *.py[co]
 /journald/*.so
+/TAGS
 
 # Packages
 *.egg

Makefile

@@ -1,6 +1,7 @@
 PYTHON = python
 SED = sed
 SPHINX_BUILD = sphinx-build
+ETAGS = etags
 INCLUDE_DIR = /usr/include/
 VERSION := $(shell $(PYTHON) setup.py --version)
 
@@ -33,4 +34,7 @@ sphinx-%: build
 	PYTHONPATH=$(builddir) $(SPHINX_BUILD) -b $* $(SPHINXOPTS) docs build/docs
 	@echo Output has been generated in build/docs
 
-.PHONY: build install dist clean distclean
+TAGS: $(shell git ls-files systemd/*.[ch])
+	$(ETAGS) $+
+
+.PHONY: build install dist clean distclean TAGS