qt5: fix clash with installed qt.

Apparently, passing HOMEBREW_PREFIX as -I and -L makes the Qt5
configure script pick up Qt4 headers. Hotfix it until I find
a solution that allows both installing to prefixes different than
/usr/local and to prefixes that already contain Qt4.

Closes #21211.

Signed-off-by: Mike McQuaid <mike@mikemcquaid.com>