lang/mono5.10: Invoke CPU_COUNT properly
3a278c7add6265e6d545f3014d7f6a6e9c222fa7 did not include a patch to
configure.ac that allowed it to properly detect CPU_COUNT.
Fixes: 3a278c7add6265e6d545f3014d7f6a6e9c222fa7
(cherry picked from commit 0435eab9f0396fceaa33339240604e740b00a858)