Fix compilation without threads support

Add missing #ifndef EVHTP_DISABLE_EVTHR
Fix compilation without regex support

Fix typo in CMakeLists.txt (DIABLE -> DISABLE) and add missing define
to compiler flags.
Fix crash when unable to bind socket with TCP deferred accept

If libevent is unable to bind to socket and USE_DEFER_ACCEPT is
configured it will segfault while trying to set TCP_DEFER_ACCEPT flag
on a socket. Fix it by checking if binding succeeded before attempting
to set the flag.
Add missing call to SSL_CTX_free() in evhtp_free().