lang/mono6.8: Invoke CPU_COUNT properly

9e02e52115e43c5c5ec3d971c9100c3cf322b726 did not include a patch to
configure.ac that allowed it to properly detect CPU_COUNT.

Fixes: 2e52115e43c5c5ec3d971c9100c3cf322b726

(cherry picked from commit ed11420d6b20eb3d10774633f70eae87c1b6e1e9)