build: Properly pass the parameters to cmake

...to build the bundled discount.

Otherwise it leads to the following cmake invocation during the build:

    'cmake DISCOUNT_ONLY_LIBRARY DISCOUNT_MAKE_INSTALL
        -DCMAKE_VERBOSE_MAKEFILE=TRUE "-GUnix Makefiles"'

Not only it doesn't do what was supposedly intended, it also causes a
hard error with cmake >= 3.23.0-rc1.