just put your library in the known shared library directory
Not good solution because I want make some changes in lib for current project.
you did before copy the files inside your sketch directory
With Makefile it's not a problem, but my co-workers using only IDE, so...
copies everything under your sketch directory into a temp directory somewhere else before compiling
I tried to use symlinks. This way:
proj/
├── sketch1
│ ├── sketch1.ino
│ ├── share -> ../share/
// sketch1.ino
#include <share/mylib.h>
gives the same ld error.
But if I create symlinks:
proj/
├── sketch1
│ ├── sketch1.ino
│ ├── mylib.h -> ../share/mylib.h
│ ├── mylib.cpp -> ../share/mylib.cpp
// sketch1.ino
#include <mylib.h>
It works. And better than suggested solutions. But it breaks compatibility with Windows.