Fix doc build dependencies for --with-system-readline

PR build/30108 concerns building gdb documentation with
--with-sytem-readline.  If the in-tree readline directory is
missing, though, the docs will fail to build:

make[4]: Entering directory '/home/keiths/work/readline-doc-issue/linux/gdb/doc'
make[4]: *** No rule to make target '../../../src/gdb/doc/../../readline/readline/doc/rluser.texi', needed by 'gdb.info'.  Stop.

The listed file (and hsuser.texi) are conditionally included by gdb.texinfo.
When system readline is used, gdb/configure.ac will leave
READLINE_TEXI_INCFLAGS empty, causing doc/Makefile.in to output a line to
$BUILD/doc/GDBvn.texi with "@set SYSTEM_READLINE".  This surpresses the
inclusion of the missing files. They are not needed or used in this
scenario.

However, GDB_DOC_SOURCE_INCLUDES always lists these two files as dependencies,
thus provoking the build error whenever readline/ is missing.

This patch fixes this by creating (essentially) a conditional setting of the
dependencies to be included from readline.

gdb/configure

@@ -746,6 +746,7 @@ LIBEXPAT
 HAVE_LIBEXPAT
 JIT_READER_DIR
 TARGET_PTR
+READLINE_DOC_SOURCE_INCLUDES
 READLINE_TEXI_INCFLAG
 READLINE_CFLAGS
 READLINE_DEPS
@@ -11449,7 +11450,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11452 "configure"
+#line 11453 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -11555,7 +11556,7 @@ else
   lt_dlunknown=0; lt_dlno_uscore=1; lt_dlneed_uscore=2
   lt_status=$lt_dlunknown
   cat > conftest.$ac_ext <<_LT_EOF
-#line 11558 "configure"
+#line 11559 "configure"
 #include "confdefs.h"
 
 #if HAVE_DLFCN_H
@@ -20793,17 +20794,20 @@ $as_echo "$gdb_cv_readline_ok" >&6; }
   READLINE_DEPS=
   READLINE_CFLAGS=
   READLINE_TEXI_INCFLAG=
+  READLINE_DOC_SOURCE_INCLUDES='$(READLINE_SYSTEM_DOC_INCLUDES)'
 else
   READLINE='$(READLINE_DIR)/libreadline.a'
   READLINE_DEPS='$(READLINE)'
   READLINE_CFLAGS='-I$(READLINE_SRC)/..'
   READLINE_TEXI_INCFLAG='-I $(READLINE_DIR)'
+  READLINE_DOC_SOURCE_INCLUDES='$(READLINE_INTREE_DOC_INCLUDES)'
 fi
 
 
 
 
 
+
 # Generate jit-reader.h
 
 # This is typedeffed to GDB_CORE_ADDR in jit-reader.h