It wasn't really a mixup exactly.

Specifying the location of `libgrib_api_f90.a` is always required, and **sometimes** (apparently) the location of `libgrib_api.a` is also requested.
Most of the time, the latter is automatically derived from the former whoever, and you just have to confirm the correctness of the derived value (which is handled by another regex).

This patch fixes the **sometimes** case, which I ran into during the regtests prior to v1.3.0.