lang/mono: Invoke CPU_COUNT properly
83c19a7238e9f5f46a9186e3ce58d03585691e5d did not include a patch to
configure.ac that allowed it to properly detect CPU_COUNT.
Fixes: 83c19a7238e9f5f46a9186e3ce58d03585691e5d
MFH: 2021Q4
(cherry picked from commit a8176108d5dd60dd740cf34a514989da21c86f99)