Use flex's -t option instead of --stdout

As reported in

  https://sourceware.org/ml/gdb-patches/2018-05/msg00042.html

some old versions of flex (2.5.4) don't support the --stdout switch.
Use -t, which is an alias.

gdb/ChangeLog:

	* Makefile.in (%.c: %.l): Use -t instead of --stdout.

gdb/ChangeLog

@@ -1,3 +1,7 @@
+2018-05-03  Simon Marchi  <simon.marchi@ericsson.com>
+
+	* Makefile.in (%.c: %.l): Use -t instead of --stdout.
+
 2018-05-03  Pedro Alves  <palves@redhat.com>
 
 	* s390-linux-nat.c

gdb/Makefile.in

@@ -2500,7 +2500,7 @@ po/$(PACKAGE).pot: force
 	  mv $@.new $@
 %.c: %.l
 	if [ "$(FLEX)" ] && $(FLEX) --version >/dev/null 2>&1; then \
-	    $(FLEX) --stdout $<  \
+	    $(FLEX) -t $<  \
 	    | sed -e '/extern.*malloc/d' \
 	        -e '/extern.*realloc/d' \
 	        -e '/extern.*free/d' \