How to supply O/S linker arguments with gprbuild?

[Warren <ve3wwg@gmail.com>]

I have a GNAT project that has these pragma statements embedded to get the sucker to link. However, the directory is only valid for an instance on my Mac, so I need a better solution to replace:

   pragma Linker_Options("-L/usr/local/Cellar/readline/8.0.4/lib");
   pragma Linker_Options("-lreadline");

I've tried to find the solution in the gnat documents but they seem to fall short of what I need to do (could not find an example nor direction in this -- perhaps I missed it).

My project file looks something like this:

project MyProject is
    for Languages use ("ada", "c");
    for Source_Dirs use ("src");
    for Object_Dir use "obj";
    for Main use ("main.adb");
end MyProject;

I've tried a variety of gpr things, but unsuccessfully. 

Warren