The documentation for hashlib++ is pretty dense, and it doesn't follow the standard
./configure
make
make install
method of compiling and installing (it doesn't have a configure script, and its Makefile is not in the top level directory). The documentation specifies that it can be included as source in your project, or compiled as a static library. From your Makefile it appears you are assuming it is compiled as a library and installed somewhere in the system's library search path. Make a note of this in the README for your project.
The documentation for hashlib++ is pretty dense, and it doesn't follow the standard
method of compiling and installing (it doesn't have a configure script, and its Makefile is not in the top level directory). The documentation specifies that it can be included as source in your project, or compiled as a static library. From your Makefile it appears you are assuming it is compiled as a library and installed somewhere in the system's library search path. Make a note of this in the README for your project.