webserver: work around a potential crash bug in libmicrohttpd 0.9.x

As observed and investigated by ulion, libmicrohttpd 0.9.x can cause crashes
on specific platforms (e.g. iOS) when using a MHD_USE_SELECT_INTERNALLY in
combination with a thread pool (MHD_OPTION_THREAD_POOL_SIZE) because of non-
copyable mutex structures.
See https://gnunet.org/bugs/view.php?id=2803 for the bug report.
Merge pull request #2316 from Montellese/libmicrohttpd_threading_workaround

webserver: work around a potential crash bug in libmicrohttpd 0.9.x