Define _STAT_VER if it's not already defined

[rgmiller]

As of glibc v2.33, _STAT_VER is no longer defined. This commit adds code to #define it to 3 if it's not already defined.

Fixes issue #687

Description

This is a very simple change: If the symbol isn't already defined, then just define it to 3.

Motivation and Context

We need this change because newer versions of glibc (starting at v2.33) no longer define the symbol. (See issue #687.)

How Has This Been Tested?

Tested on a Fedora 34 VM. (The "enterprise class" Linux distros haven't a new enough glibc yet.)

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Performance enhancement (non-breaking change which improves efficiency)
• [ ] Code cleanup (non-breaking change which makes code smaller or more readable)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Testing (addition of new tests or update to current tests)
• [ ] Documentation (a change to man pages or other documentation)

Checklist:

• [x ] My code follows the UnifyFS code style requirements.
• [N/A ] I have updated the documentation accordingly.
• [x ] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes.
• [x ] All new and existing tests passed.
• [x ] All commit messages are properly formatted.

[adammoody]

Thanks, @rgmiller