Minor autoconf tweaks

deprecate zmq_ctx_destroy in favor of zmq_ctx_term

AM_CONFIG_HEADER raises an 'obsolete error' with automake 1.13.

autotools warns that configure.in
should actually be named configure.ac

in order to avoid logical collisions with pre-existing notions of context destruction in bindings (czmq, pyzmq).

Clarified how icp:// works with zmq_bind

Fixed el5/el6 deps in RPM spec file / newline fix

Whitespace and comment cleanups

This reverts commit da0efaa8.

This reverts commit 8ba097f3.

allow XSUB/XPUB to send/recv messages unrelated to sub/unsub

zmq::xpub_t::xread_activated() – change to process messages without 0
or 1 prefix, but without affecting subscriptions

zmq::xsub_t::xsend() – change to send rather than discard messages
without 0 or 1 prefix, but without affecting subscriptions

Update documentation

Fix minor problem with cmake bulid

Make CMake build usable for other systems

Fix link errors when using -m32 with MinGW

The CPack NSIS installer was in a state where it would always give a
useless error. I think it was using stuff intended for running cpack
separately from cmake.

Fix warnings with ICC.