bitcoin: Build with (official binary installer) Qt 5.6 on OS X not supported

Qt 5.6 was released a few days ago (http://blog.qt.io/blog/2016/03/16/qt-5-6-released/).

configure --with-gui=qt5 on OS X (can’t test other systems right now) fails with:

checking for QT... no
configure: error: Qt dependencies not found

config.log contains:

configure:22230: checking for QT
configure:22237: $PKG_CONFIG --exists --print-errors "$qt5_modules"
Package Qt5Core was not found in the pkg-config search path.
Perhaps you should add the directory containing `Qt5Core.pc'
to the PKG_CONFIG_PATH environment variable
No package 'Qt5Core' found
Package Qt5Gui was not found in the pkg-config search path.
Perhaps you should add the directory containing `Qt5Gui.pc'
to the PKG_CONFIG_PATH environment variable
No package 'Qt5Gui' found

Qt 5.6 contains only 2 *.pc files compared to 89 in e.g. 5.4:

pavel$ find Qt/5.4 -name *.pc | wc -l
      89
pavel$ find Qt5.6.0 -name *.pc
Qt5.6.0/5.6/clang_64/lib/pkgconfig/Qt5OpenGLExtensions.pc
Qt5.6.0/5.6/clang_64/lib/pkgconfig/Qt5UiTools.pc
pavel$ 

Relevant upstream change: https://codereview.qt-project.org/#/c/140954/