configure.ac: Check for the readline.h explicitly

When readline development package is missing make fails with
"configure: error: system readline is not new enough" which
might be confusing. This patch checks for the readline.h explicitly
and makes make to warn about the missing package.

gdb/configure

@@ -9224,6 +9224,23 @@ fi
 
 
 if test "$with_system_readline" = yes; then
+  for ac_header in readline/readline.h
+do :
+  ac_fn_c_check_header_mongrel "$LINENO" "readline/readline.h" "ac_cv_header_readline_readline_h" "$ac_includes_default"
+if test "x$ac_cv_header_readline_readline_h" = xyes; then :
+  cat >>confdefs.h <<_ACEOF
+#define HAVE_READLINE_READLINE_H 1
+_ACEOF
+ readline_h=yes
+else
+  readline_h=no
+fi
+
+done
+
+  if test "$readline_h" = "no"; then
+    as_fn_error $? "readline development packages are probably missing" "$LINENO" 5
+  fi
   { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether system readline is new enough" >&5
 $as_echo_n "checking whether system readline is new enough... " >&6; }
 if ${gdb_cv_readline_ok+:} false; then :