How to use existing (non-arduino-specific) library with subfolders in Arduino IDE?
To utilize subfolders with any name in an Arduino library, you must use the 1.5 library specification.

In libraries folder in your sketch folder, create a folder for the library. In this folder create a src subfolder and put the library source code into that subfolder. Then create a library.properties file in the root folder of the library with some dummy information. Fill the includes key with the main .h file of the library.

Documents/
  Arduino/
   libraries/
     FooLib/
      src/
       subfoldername/
        header.h
        some.h
        some.cpp 
       foolib.h 
       foolib.cpp
      library.properties


In code then use in #include the path with src folder as starting location.

#include &lt;subfoldername/header.h&gt;