build: Fix OSX build when using Homebrew and qt5

Original PR: #5026

Qt5 is bottled, so configure won't find it without some help. Use
brew to find out its prefix.

Also, qt5 added the host_bins variable to pkg-config, use it.

@gavinandresen In order to deploy 0.9.3 successfully, I also needed to backport 502972f16bae79cf025bf9b63ec8276bc4a236bb and d16f6f87e1c15bd686e0e74ddb671da95a916c6d, though this was a quick local build and not gitian. It probably makes sense to just leave them alone if deploy/sign currently work for you, just thought I'd mention.