Or do it fast in a dirty way
#include "../libone/one.h"
or (preferably, because then it doesnβt matter if one.h is one.h either in the source tree or decomposed into another project)
#include <libone/one.h>
and in Makefile.am
libtwo_a_SOURCES = two.c two.h libtwo_a_CPPFLAGS = -I$(top_srcdir)
source share