[MPlayer-dev-eng] configure: use case instead of grep

[Reimar Döffinger]

> On 26 Feb 2022, at 20:46, Reimar Döffinger <Reimar.Doeffinger at gmx.de> wrote:
> 
> Replace the grep with a case.
> This is about 20% faster on macOS and 3x faster in WSL1 on Windows.

Applied it.