Fix stack protector check

* "return 0;" is not enough to trigger it.

Change-Id: I5a011f664aec30096cd5d26a1f94a0405297b1db
Reviewed-on: https://gerrit.libreoffice.org/58437
Tested-by: Jenkins
Reviewed-by: Michael Meeks <michael.meeks@collabora.com>

configure.ac

@@ -5923,7 +5923,7 @@ if test "$GCC" = "yes" -o "$COM_IS_CLANG" = TRUE; then
     AC_MSG_CHECKING([whether $CC supports -fstack-protector-strong])
     save_CFLAGS=$CFLAGS
     CFLAGS="$CFLAGS -Werror -fstack-protector-strong"
-    AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[ return 0; ]])],[ HAVE_GCC_STACK_PROTECTOR_STRONG=TRUE ],[])
+    AC_LINK_IFELSE([AC_LANG_PROGRAM([[]], [[ char a[8]; return 0; ]])],[ HAVE_GCC_STACK_PROTECTOR_STRONG=TRUE ],[])
     CFLAGS=$save_CFLAGS
     if test "$HAVE_GCC_STACK_PROTECTOR_STRONG" = "TRUE"; then
         AC_MSG_RESULT([yes])