Make sure docs are installed.

Index: Makefile.in
--- Makefile.in.orig
+++ Makefile.in
@@ -331,7 +331,7 @@ target_alias = @target_alias@
 top_build_prefix = @top_build_prefix@
 top_builddir = @top_builddir@
 top_srcdir = @top_srcdir@
-SUBDIRS = src
+SUBDIRS = doc src
 DIST_SUBDIRS = doc src
 EXTRA_DIST = \
     AUTHORS CODE_OF_CONDUCT COPYING HACKING INSTALL NEWS PHILOSOPHY README \
