cmake: avoid problems from use of CMAKE_USE_PTHREADS_INIT

In some CMake integrations, portions of the toolchain may end up
defining CMAKE_USE_PTHREADS_INIT even when EVENT__DISABLE_THREAD_SUPPORT
is set for libevent.  Modify the build to not rely on this side effect
of find_package(Threads, ...) [which could be done elsewhere] but instead
to use EVENT__HAVE_PTHREADS which is defined only on the desired codepath.

Without this change, affected builds fail as a result of event_pthreads source
files being built but with build defines which cause the build to fail.

Signed-off-by: Paul Osborne <paul.osborne@smartthings.com>
(cherry picked from commit 26d5ff45)