Fix QCC debug build flag

[johnmcfarlane]

Proposed changes

This change allows Boost to be built with q++ 7.1. Full details can be found in this boost issue. It essentially reimplements the patch provided by @Johnnyxy.

Types of changes

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• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Documentation content changes
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Checklist

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• I added tests that prove my fix is effective or that my feature works
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Further comments