[macOS]: When using -qtlibinfix with a prefix build then it will fail to build anything outside of Qt itself

[macOS]: When using -qtlibinfix with a prefix build then it will fail to build anything outside of Qt itself. For example doing:

./configure -qtlibinfix testinfix -prefix /usr/local/qt5.6.1
make 
sudo make install

will build fine, but when you try to build any application it will fail as it does not find the include files with something like:

In file included from main.cpp:1:
./mainwindow.h:4:10: fatal error: 'QMainWindow' file not found
#include <QMainWindow>
         ^
1 error generated.
make: *** [main.o] Error 1

whereas it will work if configured as an in-source build.