* einfo.adb (Is_External_State): An abstract state is also external
when it is declared with option "synchronous".
* einfo.ads: Update the documentation of synthesized attribute
Is_External_State.
* sem_util.adb (Find_Simple_Properties): New routine.
(Is_Enabled_External_Property): New routine.
(State_Has_Enabled_Property): Reimplemented. The two flavors of option
External have precedence over option Synchronous when determining
whether a property is in effect.
gcc/testsuite/
* gnat.dg/sync2.adb, gnat.dg/sync2.ads: New testcase.