Normally configure uses pkg-config for detecting Qt and setting
QT_CFLAGS and QT_LIBS. With this option configure won't use pkg-config
and rely on the user having set both variables before running
configure. This enables using Qt on systems where no pkg-config files
are generated (e.g. Qt 5.6.0 on MacOS with frameworks enabled).
See https://github.com/Homebrew/homebrew/pull/50234