Disable generation of documentation

Generation of documentation is not necessary in Buildroot, disable it completely.

Signed-off-by: Dmitry Golubovsky <golubovsky@gmail.com>

diff -urN a/doc/Makefile.am b/doc/Makefile.am
--- a/doc/Makefile.am	2012-08-15 06:08:43.000000000 -0500
+++ b/doc/Makefile.am	2013-07-17 09:54:53.000000000 -0500
@@ -2,8 +2,6 @@
 # Copyright (c) 2005, 2006 Thorsten Kukuk <kukuk@suse.de>
 #
 
-SUBDIRS = man specs sag adg mwg
-
 CLEANFILES = *~
 
 dist_html_DATA = index.html
@@ -11,11 +9,4 @@
 #######################################################
 
 releasedocs: all
-	$(mkinstalldirs) $(top_builddir)/Linux-PAM-$(VERSION)/doc/specs
-	cp -av specs/draft-morgan-pam-current.txt \
-		$(top_builddir)/Linux-PAM-$(VERSION)/doc/specs/
-	cp -av $(srcdir)/specs/rfc86.0.txt \
-		$(top_builddir)/Linux-PAM-$(VERSION)/doc/specs/
-	make -C sag releasedocs
-	make -C adg releasedocs
-	make -C mwg releasedocs
+	/bin/true