Do not define DIST_SUBDIRS when not needed

DIST_SUBDIRS is automatically populated with all possible values of
SUBDIRS in all conditions:

    https://www.gnu.org/software/automake/manual/html_node/SUBDIRS-vs-DIST_005fSUBDIRS.html

This means it is usually not needed to explicitely set it but in
examples/, where some dirs are never added to SUBDIRS.

Makefile.am

@@ -38,8 +38,9 @@ SUBDIRS = \
 	script
 
 if ENABLE_KERNEL
-SUBDIRS += devices
-SUBDIRS += master
+SUBDIRS += \
+	devices \
+	master
 endif
 
 if BUILD_TOOL
@@ -47,26 +48,16 @@ SUBDIRS += tool
 endif
 
 if ENABLE_USERLIB
-SUBDIRS += lib
+# userspace examples depend on lib/
+SUBDIRS += \
+	lib \
+	examples
 endif
 
 if ENABLE_TTY
 SUBDIRS += tty
 endif
 
-# userspace example depends on lib/
-SUBDIRS += examples
-
-DIST_SUBDIRS = \
-	devices \
-	examples \
-	include \
-	lib \
-	m4 \
-	master \
-	script \
-	tool \
-	tty
 
 noinst_HEADERS = \
 	globals.h

devices/Makefile.am

@@ -33,12 +33,6 @@ SUBDIRS = \
 	e1000e \
 	igb
 
-DIST_SUBDIRS = \
-	ccat \
-	e1000 \
-	e1000e \
-	igb
-
 # using HEADERS to enable tags target
 noinst_HEADERS = \
 	8139too-2.6.13-ethercat.c \

examples/Makefile.am

@@ -54,6 +54,8 @@ SUBDIRS += \
 endif
 endif
 
+# Here DIST_SUBDIRS needs to be explicitely defined because
+# dc_rtai, mini and rtai are never added to `SUBDIRS`
 DIST_SUBDIRS = \
 	dc_rtai \
 	dc_user \