CONNECT-323 Set onapp created_at and deleted_at timestamps.

configure.ac
Check for re-entrant time functions.

oaprocci/oacontract.c
Altered: "create_onapp_vm()" to set "created_at" time;
"delete_onapp_contract()" and "delete_onapp_provisioning()" to set "deleted_at" time.

oaprocci/onapp_helpers.c
Added: "attempt_gmtime_r()", "onapp_current_timestamp_int()", "onapp_current_timestamp_string()".

oaprocci/onapp_helpers.h
Added: "onapp_current_timestamp_int()", "onapp_current_timestamp_string()".