Linking files through makefiles - c++

Question

I have two directories at the same level. I want to use the header file created in one directory in the source file of the other directory. I want to achieve the same without providing relative or absolute path to the header file.
The directory structure is:
main_dir/
dir1/
main.cpp
Makefile.am
dir2/
abc.h
abc.cpp
Makefile.am
main.cpp needs abc.h. What should be written in makefiles of both the directories in order to link them?
Edit
Makefile.am of dir2
noinst_LIBRARIES = libabc.a
AM_CPPFLAG = -I$(main_dir)/dir2 \
$(APP_CFLAGS)
libabc_a_SOURCES = abc.h \
abc.cpp
Makefile.am of dir1
noinst_LIBRARIES = libmain.a
AM_CPPFLAG = -I$(main_dir)/dir1 \
$(APP_CFLAGS)
libmain_a_SOURCES = main.cpp
libmain_a_LIBADD = libabc.a
I get the error: No rule to make target 'libabc.a', needed by 'libmain.a'. Stop.

Answer 1

You can use the -I option with gcc or clang. (Note this is not directly related to makefiles)
Example: If the current directory is main_dir, and you're currently running the command:
gcc -c -o dir1/main.o dir1/main.cpp
you would need to change it to something like:
gcc -Idir2 -c -o dir1/main.o dir1/main.cpp

Answer 2

You already have
AM_CPPFLAG = -I$(main_dir)/dir1 \
$(APP_CFLAGS)
Simply add -I$(main_dir)/dir2 into it:
AM_CPPFLAG = -I$(main_dir)/dir1 \
-I$(main_dir)/dir2 \
$(APP_CFLAGS)
So now you can #include <abc.h> in your code.