#include <header.h> searches for the file in certain standard places according to implementation-defined search rules.

#include "header.h" will also search in the standard places, but usually after searching some local places, such as the current working directory.

To me, that particular error message might mean that SDCC is not aware of where its "standard places" are. Is there an environment variable that SDCC requires to tell it where its own header files are located and if so, is it set correctly or exported correctly from your makefile?