0e740d15e8
1. add -DECM_MKSPECS_INSTALL_DIR= to MSYS2_ARG_CONV_EXCL so mkspecs files are placed in /mingw{32,64} instead of /msys{32,64}/mingw{32,64}
2. change deprecated LIB_INSTALL_DIR with KDE_INSTALL_LIBDIR
3. place mkspecs files in ${QT5_PREFIX}/share/qt5/mkspecs (Qt5 mkspecs dir) instead of ${QT5_PREFIX} which points to root /mingw{32,64}