Hopefully, fix the long-standing problem with h2root

[couet]

Using the C option (meaning C I.O) in hropen seems to fix the issue on Mac.

[github-actions[bot]]

Test Results

    18 files      18 suites   4d 11h 9m 38s ⏱️  2 682 tests  2 680 ✅ 0 💤 2 ❌ 46 556 runs  46 553 ✅ 0 💤 3 ❌

For more details on these failures, see this check.

Results for commit e0527581.

:recycle: This comment has been updated with latest results.

[hahnjo]

As far as I can tell, this shouldn't make a difference because HROPEN defaults to to C unless F is specified: https://github.com/root-project/root/blob/80023e019a91244d63a3ee961b8b9a84afb4fe7e/misc/minicern/src/hbook.f#L260-L263

[couet]

@hahnjo it made on on MacOS. (long debug)

[hahnjo]

my comment from https://github.com/root-project/root/pull/15915#issuecomment-2188477214 still stands: based on the code, there should be no change in behavior because C is already the default.

[couet]

I know "c" is supposed to be the default, but this change made a difference on Mac when I debugged it a while ago. I guess it is worth merging.

[ferdymercury]

h2root failure is also visible in CI on Ubuntu25 (https://github.com/root-project/root/actions/runs/15393615777/job/43308659605?pr=18918)

[ferdymercury]

Continued at https://github.com/root-project/root/pull/19191