Fixes #3882: Move r29588, r29597 to v1.7 branch

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r29588 [[BR]]
Fix installation of libevent header files with --with-devel-headers isn't specified

cmr=v1.7.4:reviewer=rhc

r29597 [[BR]]
Add an Open MPI-specific comment here so that we hopefully don't lose
this change the next time we update libevent.

cmr=v1.7.4:ticket=3882