Fix clash with installed Qt4

Apparently, passing HOMEBREW_PREFIX as -I and -L makes the Qt 5
configure script pick up Qt 4 headers. (see https://gist.github.com/mbcoguno/fb4ead7747822a0ff83b).
Hotfix it until I find
a solution that allows both installing to prefixes different than
/usr/local and to prefixes that already contain Qt 4.