Check that CMake flag is set before prepending an absolute path

[yourslab]

This catches the corner case where the user has not passed a CMake flag but the absolute path logic is still applied.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov-commenter]

Codecov Report

Merging #1127 into development will increase coverage by 2.30%.
The diff coverage is 100.00%.

@@               Coverage Diff               @@
##           development    #1127      +/-   ##
===============================================
+ Coverage        96.54%   98.85%   +2.30%     
===============================================
  Files                9        4       -5     
  Lines             5643     1309    -4334     
  Branches           641      393     -248     
===============================================
- Hits              5448     1294    -4154     
+ Misses               9        0       -9     
+ Partials           186       15     -171     

Impacted Files	Coverage Δ
libraries/standard/mqtt/src/mqtt.c	99.48% <ø> (+5.06%)	⬆️
libraries/standard/mqtt/src/mqtt_lightweight.c	97.42% <ø> (+2.90%)	⬆️
libraries/standard/mqtt/src/mqtt_state.c	98.46% <ø> (+2.52%)	⬆️
libraries/standard/http/src/http_client.c	100.00% <100.00%> (+12.51%)	⬆️

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