lang/mono5.20: Invoke CPU_COUNT properly

6017a183afd1d58656a75f72277f0cd5111cc0e6 did not include a patch to
configure.ac that allowed it to properly detect CPU_COUNT.

Fixes: 6017a183afd1d58656a75f72277f0cd5111cc0e6

(cherry picked from commit 6e22c3c3fd29ac31c0611e29e6068b62c1e89b43)