Move from legacy -levent to recommended -levent_core

* `libevent_core.so` contains the core functionality that we depend upon
   - `libevent.so` library has been identified as the legacy target.
   - `libevent_core.so` exists as far back as Libevent 2.0.5 (oldest supported by OMPI)
 * `libevent_pthreads.so` can work with either `-levent` or `-levent_core`

Signed-off-by: Joshua Hursey <jhursey@us.ibm.com>
(cherry picked from commit 886f41fe3381a338eac215f26360980c612e6bb8)

opal/mca/event/external/configure.m4

@@ -108,7 +108,7 @@ AC_DEFUN([MCA_opal_event_external_CONFIG],[
 
            OPAL_CHECK_PACKAGE([opal_event_external],
                               [event2/event.h],
-                              [event],
+                              [event_core],
                               [event_config_new],
                               [-levent_pthreads],
                               [$opal_event_dir],
@@ -150,7 +150,7 @@ AC_DEFUN([MCA_opal_event_external_CONFIG],[
                  [# Ensure that this libevent has the symbol
                   # "evthread_set_lock_callbacks", which will only exist if
                   # libevent was configured with thread support.
-                  AC_CHECK_LIB([event], [evthread_set_lock_callbacks],
+                  AC_CHECK_LIB([event_core], [evthread_set_lock_callbacks],
                                [],
                                [AC_MSG_WARN([External libevent does not have thread support])
                                 AC_MSG_WARN([Open MPI requires libevent to be compiled with])