math/gretl: fix build without OpenMP

Remove calls to OpenMP functions when OpenMP is disabled.
Disable OpenMP where not available.
Remove bogus -lomp link flag. Port now builds fine on armv7.

Approved by: portmgr (build fix blanket)
MFH: 2026Q3

(cherry picked from commit 907de55ddfdbd896c1d1444fd0a65973259b2412)