Including header files which are in a sub directory

I am currently working on porting a AVR C library into Arduino library. The AVR C library has header files in separate directories and are include using relative path.

Now to port the library to Arduino, I have copied all the files including the sub-directories into a separate folder inside the /libraries folder. But when I try to include the header files in the subdirectory I am not able to include them.

Is there an alternate way to include those files, apart from copying them into the parent directory?

I have copied all the files including the sub-directories into a separate folder inside the /libraries folder

Works for me.