qt3d: OSX: assimp config test doesn't pass

Brew includes assimp and pkg-config. Of course brew can't be a requirement for building Qt on OS X, but it would be nice if assimp could be found in this case.

If I change ~/dev/qt5/qt3d/config.tests/assimp/assimp.pro:

-LIBS += -lassimp
+LIBS += -lassimp -L/usr/local/lib

-unix {
     CONFIG += link_pkgconfig
     PKGCONFIG += assimp
-}

so that it should try to use pkgconfig on all OSes, it still says

Project ERROR: assimp development package not found