[Development] Proposal: Use -qt-zlib configuration in official MinGW builds

[Konstantin Tokarev]

Hello,

Currently, MinGW builds in Coin use -system-zlib configuration. It happens
because MinGW is shipped with zlib headers and libz.a. However, linking zlib
to several Qt modules (at least, QtCore, QtGui, QtNetwork, and QtSvg) is
suboptimal, while with -system-zlib one copy in QtCore is shared between all
modules.

Also this would be consistent with MSVC builds.

-- 
Regards,
Konstantin