Rename event_extras to event_extra

The library event_extras is named event_extra when building with
GNU autotools
Add option to build shared library
Add -Qunused-arguments for clang on macos

The clang compiler provided with macosx emits warnings like:

  CC       bufferevent.lo
clang: warning: argument unused during compilation: '-I .'
clang: warning: argument unused during compilation: '-I ./compat'
clang: warning: argument unused during compilation: '-I ./include'
clang: warning: argument unused during compilation: '-I ./include'

for each file being compiled. This generates a lot of noise during
compilation making it hard to see "real" errors. This patch mute
those warnings.
Merge remote-tracking branch 'origin/patches-2.0'
Merge remote-tracking branch 'origin/pr/110'
Move assert(ev) to before we use ev in EV_CLOSURE_EVENT_FINALIZE case

Based on a patch from Harlan Stenn.
Fix 'make distcheck' by adding regress.gen.[ch] to DISTCLEANFILES

Patch from Harlan Stenn.
Update to the latest version of tinytest

This brings us up to tinytest 709a36ba63ff16d8
Export event_extra not event_extras.
add a cast to https-client.c
Initialize async bufferevent timeout CBs unconditionally

This is a fix for bug #93 on nmathewson/Libevent.
Fix duplicate paragraph in evbuffer_ptr documentation
Fix consts in WIN32-Code/getopt*.[ch]
Heap-allocate zlib data structure in regress_zlib tests