Add CXXFLAGS to unifyfs-ls

[CamStan]

Description

Add -std=c++11 CXXFLAGS to the unifyfs-ls target in order to build on GCC v4.9.3.

Motivation and Context

Fixes #738

How Has This Been Tested?

Built successfully locally with GCC v4.9.3

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.
• [ ] 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.

[CamStan]

ping @rgmiller. For some reason it won't let me request you as a reviewer.

[rgmiller]

Hmm... It's odd that adding -std=c++11 also fixed the problems with free() and exit() not being found.