[mathmore] make GSLinterpolator Eval error printing thread-safe

[ferdymercury]

This Pull request:

Changes or fixes:

Fixes https://its.cern.ch/jira/browse/ROOT-8725

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    19 files      19 suites   5d 5h 51m 50s ⏱️
 2 729 tests  2 727 ✅ 0 💤 2 ❌
50 043 runs  50 041 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit 5568dab.

[pcanal]

Thanks.