mu4e: bump minimal required emacs to 24.4

We didn't really support older versions anyway, so let's make it
official.

Makefile.am

@@ -1,4 +1,4 @@
-## Copyright (C) 2008-2013 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+## Copyright (C) 2008-2018 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 ##
 ## This program is free software; you can redistribute it and/or modify
 ## it under the terms of the GNU General Public License as published by
@@ -52,7 +52,7 @@ tags:
 # which is a sign that it needs some refactoring. requires the pmccabe
 # tool. If all is fine, it outputs nothing
 cc10:
-	@$(PMCCABE) `find . -name '*.c' -o -name '*.cc'`		\
+	@$(PMCCABE) `find . -name '*.c' -o -name '*.cc'`	\
 	| grep -v mu-str-normalize.c				\
 	| grep -v mu_str_subject_normalize			\
 	| grep -v tests						\
@@ -77,20 +77,20 @@ fixme:
 
 # check whether we can run make distcheck from the repo version
 gitcheck:
-	cd `mktemp -d`;				 \
-	git clone git://github.com/djcb/mu.git ; \
-	cd mu;					 \
-	autoreconf -i ;				 \
-	./configure ;				 \
+	cd `mktemp -d`;						\
+	git clone git://github.com/djcb/mu.git ;		\
+	cd mu;							\
+	autoreconf -i ;						\
+	./configure ;						\
 	make distcheck
 
-EXTRA_DIST=		\
-	TODO		\
-	HACKING		\
-	gtest.mk	\
-	NEWS		\
-	NEWS.org	\
+EXTRA_DIST=							\
+	TODO							\
+	HACKING							\
+	gtest.mk						\
+	NEWS							\
+	NEWS.org						\
 	autogen.sh
 
-doc_DATA =			\
+doc_DATA =							\
 	NEWS.org