How to link using GCC without -l nor hardcoding path for a library that does not follow the libNAME.so naming convention?

There is the ":" prefix that allows you to give different names to your libraries. If you use

g++ -o build/bin/myapp -l:_mylib.so other_source_files

should search your path for the _mylib.so.

If you can copy the shared library to the working directory when g++ is invoked then this should work:

g++ -o build/bin/myapp _mylib.so other_source_files