Changed Makefile so opt forces the gnu17 standard

[RoryBarnes]

Proposed changes

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[github-actions]

Test Results

19 599 tests   19 599 ✅  1m 34s ⏱️
     1 suites       0 💤
     1 files         0 ❌

Results for commit 8450e74.

♻️ This comment has been updated with latest results.