Makefile: Fix Makefiles to depend on all child Makefile fragments.

We were previously maintaining two lists of the child Makefile
fragments---one for the includes and another for the dependencies. So,
of course, they drifted and the dependency list wasn't up to date.

We fix this by adding a single subdirs variable, and then using GNU
Makefile substitution to generate both the include and the dependency
lists.

Some side effect of this change caused the '=' assignment of the dir
variable to not work anymore. I'm not sure why that is, but using ':='
makes sense here and fixes the problem.

Makefile

@@ -10,9 +10,6 @@ gzip = gzip
 bash_completion_dir = /etc/bash_completion.d
 zsh_completion_dir = /usr/share/zsh/functions/Completion/Unix
 
-global_deps = Makefile Makefile.local Makefile.config \
-		   lib/Makefile lib/Makefile.local
-
 extra_cflags :=
 extra_cxxflags :=
 
@@ -44,12 +41,12 @@ Makefile.config: configure
 	@echo ""
 	./configure
 
-include Makefile.config
+subdirs = compat emacs lib
 
-include lib/Makefile.local
-include compat/Makefile.local
-include emacs/Makefile.local
-include Makefile.local
+global_deps = Makefile Makefile.config Makefile.local \
+	$(subdirs:%=%/Makefile) $(subdirs:%=%/Makefile.local)
+
+include Makefile.config Makefile.local $(subdirs:%=%/Makefile.local)
 
 # The user has not set any verbosity, default to quiet mode and inform the
 # user how to enable verbose compiles.

compat/Makefile.local

@@ -1,4 +1,4 @@
-dir=compat
+dir := compat
 extra_cflags += -I$(dir)
 
 notmuch_compat_srcs =

emacs/Makefile.local

@@ -1,8 +1,7 @@
-dir=emacs
-emacs_sources=                 \
-	$(dir)/notmuch.el
+dir := emacs
+emacs_sources := $(dir)/notmuch.el
 
-emacs_bytecode=$(subst .el,.elc,$(emacs_sources))
+emacs_bytecode := $(subst .el,.elc,$(emacs_sources))
 
 emacs: $(emacs_bytecode)
 

lib/Makefile.local

@@ -1,4 +1,4 @@
-dir=lib
+dir := lib
 extra_cflags += -I$(dir)
 
 libnotmuch_c_srcs =		\