Makefile: Add cscope and tags rules

These were added to the kernel repo, but not in Github. However, they are
useful for browsing the source in Github while prototyping new features and
compiling them into userspace utilities.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>

src/Makefile

@@ -68,6 +68,8 @@ LIBDIR ?= $(PREFIX)/$(LIBSUBDIR)
 INCLUDEDIR ?= $(PREFIX)/include
 UAPIDIR ?= $(PREFIX)/include
 
+TAGS_PROG := $(if $(shell which etags 2>/dev/null),etags,ctags)
+
 all: $(STATIC_LIBS) $(SHARED_LIBS) $(PC_FILE)
 
 $(OBJDIR)/libbpf.a: $(STATIC_OBJS)
@@ -133,3 +135,12 @@ install_pkgconfig: $(PC_FILE)
 
 clean:
 	rm -rf *.o *.a *.so *.so.* *.pc $(SHARED_OBJDIR) $(STATIC_OBJDIR)
+
+.PHONY: cscope tags
+cscope:
+	ls *.c *.h > cscope.files
+	cscope -b -q -f cscope.out
+
+tags:
+	rm -f TAGS tags
+	ls *.c *.h | xargs $(TAGS_PROG) -a