Fixed incorrect QXT_INSTALL_FEATURES initialization with Qt5 on Windows.

The Windows command processor substitutes variables' values all at once before it executes the next line in batch file. This leads to incorrect initialization of QXT_INSTALL_FEATURES - it is updated twice in single line, and the second update ignores the result of the first, though it should to depend on it. It is fixed by enabling delayed variables' values expansion (see diff and comments inside batch file).