Hello,
From a Qt Creator 4.1.0 Based on Qt 5.7.0 (GCC 4.9.1 20140922 (Red Hat 4.9.1-10), 64 bit),
I create a library: File > New File or Project > Library > C++ Library
called "MathFunctions" and does't touch a file. So there are the skeleton files, and build them, with the release and debug flavors; all going well without errors.

Now I create another project: File > New File or Project > Application > Qt Console Application: "MathFunctionsTest"
After that, right click over the project "MathFunctionsTest" and: Add library > External library

selected: Library file; Include path; Linux
MathFunctionsTest.pro ends up with this snippet inserted:

Qt Code:
  1. unix:!macx: LIBS += -L$$PWD/../../MathFunctions/build-MathFunctions-Desktop_Qt_5_7_0_GCC_64bit-Release/ -lMathFunctions
  2.  
  3. INCLUDEPATH += $$PWD/../../MathFunctions/build-MathFunctions-Desktop_Qt_5_7_0_GCC_64bit-Release
  4. DEPENDPATH += $$PWD/../../MathFunctions/build-MathFunctions-Desktop_Qt_5_7_0_GCC_64bit-Release
  5.  
  6. unix:!macx: PRE_TARGETDEPS += $$PWD/../../MathFunctions/build-MathFunctions-Desktop_Qt_5_7_0_GCC_64bit-Release/libMathFunctions.a
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Now I go to main.cpp and try to add:

#include "MathFunctions"
Qt creator highlights with "no such file", but directories and files do exist.

Please, any ideas?

Regards