m4: improve help message of libo_CHECK_SYSTEM_MODULE

Change-Id: I784823b27108671e6bb549f60725f21abd47451e

m4/libo_externals.m4

@@ -9,7 +9,7 @@ dnl -*- Mode: Autoconf; tab-width: 4; indent-tabs-mode: nil; fill-column: 102 -*
 AC_DEFUN([libo_CHECK_SYSTEM_MODULE], [
 AC_ARG_WITH(system-$1,
     AS_HELP_STRING([--with-system-$1],
-        [Use $1 already on system.]),,
+        [Use $1 from operating system instead of building and bundling it.]),,
     [with_system_$1="$with_system_libs"])
 AC_MSG_CHECKING([which $1 to use])
 if test "$with_system_$1" = "yes"; then