Enable C99 in CentOS 7 and SLES for building Git
Enable C99 in CentOS 7 and SLES for building Git

Git v2.35 added a balloon test for C99 support
(https://gitlab.com/gitlab-org/gitlab-git/-/commit/7bc341e21b5).

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.

Relates to https://gitlab.com/gitlab-org/gitaly/-/issues/4082

Changelog: fixed

gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab,gitlabhq/omnibus-gitlab