Fix configure error regarding find_package which cannot find qmsetup

[fragoulin]

When using a mxe toolchain, the following error appears:

CMake Error at build/win64/_deps/windowkit-src/CMakeLists.txt:133 (find_package):
  Could not find a package configuration file provided by "qmsetup" with any
  of the following names:

    qmsetupConfig.cmake
    qmsetup-config.cmake

  Add the installation prefix of "qmsetup" to CMAKE_PREFIX_PATH or set
  "qmsetup_DIR" to a directory containing one of the above files.  If
  "qmsetup" provides a separate development package or SDK, be sure it has
  been installed.

This fix simply sets the qmsetup_DIR variable before the call to find_package.

Related to #195