# mkdir src/testmodule
# touch src/testmodule/testfunc.cpp
# touch src/testmodule/testfunc.h
# touch src/testmodule/Makefile.am

src/testmodule/testfunc.h

void printMessage();

src/testmodule/testfunf.cpp

#include <stdio.h>
#include "testfunc.h"

void printMessage()
{
	printf("hello world!\n");
}

src/testmodule/Makefile.am

noinst_LIBRARIES = libtestmodule.a
noinst_HEADERS = testfunc.h
libtestmodule_a_SOURCES = testfunc.cpp

src/main.cpp

#include <stdio.h>
#include "testproj.h"
#include "testmodule/testfunc.h"

int main(int)
{
	printMessage();
}

src/Makefile.am

SUBDIRS = testmodule

bin_PROGRAMS = testproj
testproj_SOURCES = main.cpp
noinst_HEADERS = testproj.h
testproj_LDADD = $(top_srcdir)/src/testmodule/libtestmodule.a

configure.ac

...
AM_INIT_AUTOMAKE
AC_PROG_RANLIB

# Checks for programs.

...

# Checks for library functions.
AC_CONFIG_FILES([Makefile src/Makefile src/testmodule/Makefile])
AC_OUTPUT

# gmake
cd . && aclocal19

...

 cd . && automake19 --gnu
cd . && autoconf259
/usr/local/bin/bash ./config.status --recheck
running /usr/local/bin/bash ./configure   --no-create --no-recursion

...

config.status: executing depfiles commands
cd . && autoheader259
rm -f stamp-h1
touch config.h.in
cd . && /bin/sh ./config.status config.h
config.status: creating config.h
config.status: config.h is unchanged
gmake  all-recursive
gmake[1]: Entering directory `/usr/home/he/develop/testproj'
Making all in src
gmake[2]: Entering directory `/usr/home/he/develop/testproj/src'
Making all in testmodule
gmake[3]: Entering directory `/usr/home/he/develop/testproj/src/testmodule'
if g++ -DHAVE_CONFIG_H -I. -I. -I../..     -g -O2 -MT testfunc.o -MD -MP -MF ".deps/testfunc.Tpo" 
	-c -o testfunc.o testfunc.cpp; \
then mv -f ".deps/testfunc.Tpo" ".deps/testfunc.Po"; else rm -f ".deps/testfunc.Tpo"; exit 1; fi
rm -f libtestmodule.a
ar cru libtestmodule.a testfunc.o
ranlib libtestmodule.a
gmake[3]: Leaving directory `/usr/home/he/develop/testproj/src/testmodule'
gmake[3]: Entering directory `/usr/home/he/develop/testproj/src'
if g++ -DHAVE_CONFIG_H -I. -I. -I..     -g -O2 -MT main.o -MD -MP -MF ".deps/main.Tpo" -c -o 
	main.o main.cpp; \
then mv -f ".deps/main.Tpo" ".deps/main.Po"; else rm -f ".deps/main.Tpo"; exit 1; fi
g++  -g -O2   -o testproj  main.o ../src/testmodule/libtestmodule.a
gmake[3]: Leaving directory `/usr/home/he/develop/testproj/src'
gmake[2]: Leaving directory `/usr/home/he/develop/testproj/src'
gmake[2]: Entering directory `/usr/home/he/develop/testproj'
gmake[2]: Leaving directory `/usr/home/he/develop/testproj'
gmake[1]: Leaving directory `/usr/home/he/develop/testproj'

# gmake install
Making install in src
gmake[1]: Entering directory `/usr/home/he/develop/testproj/src'
Making install in testmodule
gmake[2]: Entering directory `/usr/home/he/develop/testproj/src/testmodule'
gmake[3]: Entering directory `/usr/home/he/develop/testproj/src/testmodule'
gmake[3]: Nothing to be done for `install-exec-am'.
gmake[3]: Nothing to be done for `install-data-am'.
gmake[3]: Leaving directory `/usr/home/he/develop/testproj/src/testmodule'
gmake[2]: Leaving directory `/usr/home/he/develop/testproj/src/testmodule'
gmake[2]: Entering directory `/usr/home/he/develop/testproj/src'
gmake[3]: Entering directory `/usr/home/he/develop/testproj/src'
test -z "/usr/local/bin" || /usr/home/he/develop/testproj/install-sh -d "/usr/local/bin"
  /usr/bin/install -c 'testproj' '/usr/local/bin/testproj'

...

# testproj
hello world!