Makefile: include flag to generate Emacs-compatible TAGS file

Emacs exposes an environment variable `INSIDE_EMACS` which can be used
to toggle ctags to generate an Emacs-friendly TAGS file.

(cherry picked from commit 35c846b1e8dc7075b0cee196a430dd1b09a87446)

Makefile

@@ -21,6 +21,10 @@ endif
 endif
 
 MKTAGS?=ctags
+EMACS_COMPAT=
+ifneq (INSIDE_EMACS,)
+EMACS_COMPAT=-e
+endif
 
 # forward all named targets
 %:
@@ -38,7 +42,7 @@ install:
 .PHONY: TAGS
 .PHONY: tags
 TAGS tags:
-	$(MKTAGS) --exclude="misc/*" --exclude="test/*" -R .
+	$(MKTAGS) $(EMACS_COMPAT) --exclude="misc/*" --exclude="test/*" -R .
 
 # clean everything generated - shortcut on maintainer-clean
 .PHONY: pure