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.