Add support for GNU GLOBAL.

Makefile

Index: Makefile
diff --git a/Makefile b/Makefile
index ff01c7d779ab5f863bea679078425db0f0b72574..16d8ef2d4506b984ecd6f901e65cfce40d38b421 100644
--- a/Makefile
+++ b/Makefile
@@ -409,7 +409,7 @@ native.clean:
 	rm -rf $(OUTDIR)/native
 	find $(OUTDIR) -regex '.*\(host\|target\)\.native\.mk' -delete
 
-clean: $(addsuffix .clean, $(ARCHES) $(ANDROID_ARCHES) $(NACL_ARCHES)) native.clean
+clean: $(addsuffix .clean, $(ARCHES) $(ANDROID_ARCHES) $(NACL_ARCHES)) native.clean gtags.clean
 
 # GYP file generation targets.
 OUT_MAKEFILES = $(addprefix $(OUTDIR)/Makefile.,$(BUILDS))
@@ -467,6 +467,16 @@ grokdump: ia32.release
 	@cat $(DUMP_FILE).tmpl > $(DUMP_FILE)
 	@$(OUTDIR)/ia32.release/d8 --dump-heap-constants >> $(DUMP_FILE)
 
+# Support for the GNU GLOBAL Source Code Tag System.
+gtags.files: $(GYPFILES) $(ENVFILE)
+	@find include src test -name '*.h' -o -name '*.cc' -o -name '*.c' > $@
+
+GTAGS GRTAGS GPATH: gtags.files
+	@GTAGSFORCECPP=yes gtags -i -q
+
+gtags.clean:
+	rm -f GTAGS GRTAGS GPATH gtags.files
+
 # Dependencies.
 # Remember to keep these in sync with the DEPS file.
 dependencies: