Enable C99 in CentOS 7 for building Git

What does this MR do?

Git v2.35 added a balloon test for C99 support (gitlab-git@7bc341e2).

This test caused builds to fail in CentOS 7, since this uses gcc v4.8.5, which uses C90 (-std=gnu90) by default. C11 is a newer standard than C90, and gcc v5.1.0 switched the default from std=gnu90 to std=gnu11.

On other platforms, C11 may be required, so to fix the CentOS 7 issue we only modify CFLAGS for CentOS 7.

Test build: https://dev.gitlab.org/gitlab/omnibus-gitlab/-/merge_requests/212

Successful CentOS 7 build: https://dev.gitlab.org/gitlab/omnibus-gitlab/-/jobs/12085654

Related issues

Relates to gitaly#4082 (closed)

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion

Required

• Merge Request Title, and Description are up to date, accurate, and descriptive
• MR targeting the appropriate branch
• MR has a green pipeline on GitLab.com
• Pipeline is green on dev.gitlab.org if the change is touching anything besides documentation or internal cookbooks
• trigger-package has a green pipeline running against latest commit

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes
• Documentation created/updated
• Tests added
• Integration tests added to GitLab QA
• Equivalent MR/issue for the GitLab Chart opened