Add gtags option to use ctags. Hidden option in Makefile

Change-Id: I6b01c1f2beec37d7aaad1ab9dab303835a9feb49
Signed-off-by: Keith Burns (alagalah) <alagalah@gmail.com>
diff --git a/Makefile b/Makefile
index 94ba90d..ad60476 100644
--- a/Makefile
+++ b/Makefile
@@ -219,6 +219,9 @@
 	@ctags --totals --tag-relative -L $<
 	@rm $<
 
+gtags: ctags
+	@gtags --gtagslabel=ctags
+
 cscope: cscope.files
 	@cscope -b -q -v