Fix building with Qt5 on MSVC by undefining min/max macros

The Windows stdlib.h header defines min/max macros, which interfer with
the Qt5 header files. Luckily, it also provides the define NOMINMAX which
suppresses these macros, and fix building.
Fix linking against Qt5 on MSVC

QT_QTMAIN_LIBRARY is no longer automatically set on Qt5, as it is part of
a deprecated way to link against Qt. Since Quassel needs to support both
Qt4 and Qt5, setting the variable to the appropriate value again works
around the issue until such a time that the CMake requirement is increased
to make use of policy CMP0020.

Note that this is the same workaround cmake itself uses when building
cmake-gui for Windows.
Merge pull request #69 from Nevcairiel/qt5msvcbuild

Qt5 MSVC build fixes