I have a program that uses some files in different folders. I won't post the entire program because it's big, but suffice to say I get this error when compiling (I use g++ with cygwin):
GirGS4.cpp:24:23: fatal error: arprec/mp.h: No such file or directory
where GirGS4.cpp is my program (and then a bunch of similar errors below). I checked that things are in the right place by making another program (in the same folder as GirGS4.cpp) which writes to arprec/test, and this file pops up in the same folder as mp.h.
Have you set a search directory that leads to the headers/sources in your IDE? In other words, have you told the compiler/linker where to look when #including your files?