Build fails in the absence of libunwind

[PhilMiller]

Describe the bug

On a system that doesn't have libunwind installed, building a project that incorporates vt leads to a compilation failure for missing libunwind.h

This will block integration of updated vt into EMPIRE.

#1657 (comment)

To Reproduce
Steps to reproduce the behavior:

1. Example/test/snippet of code that fails
2. Compiler, platform, libraries
3. Run command: number of processors, threading options, etc.
4. See error

Expected behavior
A clear and concise description of what you expected to happen.

Screenshots
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Platform (please complete the following information):

• Supercomputer or desktop [e.g., Trinity, Sierra)
• Compiler environment [e.g., clang-3.9, gcc8]
• Libraries [e.g., mpich-2, openmpi)
• Commit ID or release of VT [e.g., vt-1.0.0]

Additional context
Add any other context about the problem here.

[cz4rs]

DARMA weekly conclusion:
Preferred solution is to check for libunwind at CMake level, fallback to libexecinfo when not available and just print minimal information (only addresses?) when nothing is available.