How to let Particle Dev Compile ignore folders/files from library?

In my Particle Photon project I added a custom library, like this:


    • (folder) lib
      • custom_library
        • LICENSE
        • makefile (to compile unittests in tests/)
        • (folder) tests
          • (folder) UnitTest++
          • Main.cpp
          • FirstTest.cpp
      • (folder) examples
        • (folder) usage
          • usage.ino
    • (folder) src
      • project.ino

When I try to build my project, I get the following errors:

~/Config.h: No such file or directory (/src/TimeHelpers.h) **

TimeHelpers.h is part of the UnitTest++ folder, which needs to be excluded or ignored by the compiler.
When I remove the ‘tests’ folder, it works.
How do I exclude/ignore the tests folder for the compiler?