build/win32: Fix 'make -jN dist'

build/win32: Fix 'make -jN dist'

The recent changes to build/win32/vs9|10/Makefile.am fixed 'make distclean'
but broke 'make -jN dist', so fix that by listing the *.headers and using
that list as a dependency and to remove those files in one single command
right after we generate the gtk-install.vsprops template, so that we don't
have to worry about them in 'make distclean'.