build: Fix OSX build when using Homebrew and qt5

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.
build: macdeploy: filter out irrelevant qt5 frameworks and dylibs
Merge pull request #5026

dd367ff build: macdeploy: filter out irrelevant qt5 frameworks and dylibs (Cory Fields)
9fedafb build: Fix OSX build when using Homebrew and qt5 (Cory Fields)

Signed-off-by: Gavin Andresen <gavinandresen@gmail.com>