build: Move 'Makefile' fragments to subdirectories.

This follows a convention used by some other GNU packages like Autoconf,
Bison, Coreutils, and Gnulib.

* doc.am: Rename to ...
* doc/local.mk: ... this.
* emacs.am: Rename to ...
* emacs/local.mk: ... this.
* gnu-system.am: Rename to ...
* gnu/local.mk: ... this.
* daemon.am: Rename to ...
* nix/local.mk: ... this.
* Makefile.am: Adapt to them.
* doc/guix.texi (Porting to a New Platform): Adapt documentation.
* guix/config.scm.in (%state-directory, %config-directory): Adapt comments.
* emacs/guix-config.el.in (guix-config-state-directory): Likewise.

Makefile.am

@@ -27,7 +27,7 @@ nodist_noinst_SCRIPTS =				\
   pre-inst-env					\
   test-env
 
-include gnu-system.am
+include gnu/local.mk
 
 MODULES =					\
   guix/base32.scm				\
@@ -416,11 +416,11 @@ install-data-hook: set-bootstrap-executable-permissions
 SUBDIRS = po/guix po/packages
 BUILT_SOURCES =
 
-include doc.am
+include doc/local.mk
 
 if BUILD_DAEMON
 
-include daemon.am
+include nix/local.mk
 
 endif BUILD_DAEMON
 
@@ -437,7 +437,7 @@ AM_DISTCHECK_CONFIGURE_FLAGS =			\
 
 dist_emacsui_DATA = emacs/guix-main.scm
 nodist_emacsui_DATA = emacs/guix-helper.scm
-include emacs.am
+include emacs/local.mk
 
 # The self-contained tarball.
 guix-binary.%.tar.xz:

doc/guix.texi

@@ -11286,7 +11286,7 @@ to be updated to refer to these binaries on the target platform.  That
 is, the hashes and URLs of the bootstrap tarballs for the new platform
 must be added alongside those of the currently supported platforms.  The
 bootstrap Guile tarball is treated specially: it is expected to be
-available locally, and @file{gnu-system.am} has rules do download it for
+available locally, and @file{gnu/local.mk} has rules do download it for
 the supported architectures; a rule for the new platform must be added
 as well.
 

emacs/guix-config.el.in

@@ -24,7 +24,7 @@
   (replace-regexp-in-string "${prefix}" "@prefix@" "@emacsuidir@"))
 
 (defconst guix-config-state-directory
-  ;; This must match `NIX_STATE_DIR' as defined in `daemon.am'.
+  ;; This must match `NIX_STATE_DIR' as defined in `nix/local.mk'.
   (or (getenv "NIX_STATE_DIR") "@guix_localstatedir@/guix"))
 
 (defconst guix-config-guile-program "@GUILE@"

guix/config.scm.in

@@ -55,11 +55,11 @@
       "@storedir@"))
 
 (define %state-directory
-  ;; This must match `NIX_STATE_DIR' as defined in `daemon.am'.
+  ;; This must match `NIX_STATE_DIR' as defined in `nix/local.mk'.
   (or (getenv "NIX_STATE_DIR") "@guix_localstatedir@/guix"))
 
 (define %config-directory
-  ;; This must match `NIX_CONF_DIR' as defined in `daemon.am'.
+  ;; This must match `NIX_CONF_DIR' as defined in `nix/local.mk'.
   (or (getenv "NIX_CONF_DIR") "@guix_sysconfdir@/guix"))
 
 (define %guix-register-program