Newer versions of Qt (definitely 5.7.0 but not yet 5.5.1) contain part
of the platform-specific code not in e.g. libqwindows.a but in
libQt5PlatformSupport.a. That library doesn't seem to be mentioned in
the LDFLAGS gathered from pkg-config. But its functionality is actually
required for libqwindows.a.
Therefore look for the library manually and link it if it's found.
Required in at least the static mingw cross-compiled build.
