Under MinGW, QMAKE_COPY_DIR wrongly uses xcopy

When having a .pro file like the following:

TEMPLATE = app
themes.path = /usr/share/mythtv/themes
themes.files = blue
INSTALLS += themes
SOURCES += dummy.c

and doing make install

make install
c:/msys/1.0/qt-win-opensource-src-4.3.4/bin/qmake.exe -win32 -o Makefile themes.pro
xcopy /s /q /y /i c:\mythtv\mythtv\themes\blue /usr/share/mythtv/themes/blue
Invalid number of parameters
make: [install_themes] Error 4 (ignored)

DOS xcopy is being used, but the correct command would be

cp -r blue /usr/share/mythtv/themes/blue