Add LLDB_LIB_PATH for overriding --libdir

[siggsy]

Fixes #22.

Some systems are not FHS compliant and therefore do not store liblldb.so in llvm-config's libdir folder. They are not FHS compliant for a reason: they have their unique way of providing packages and libraries to avoid conflicts and to allow fully producible environments (eg: NixOS, guix).

This PR helps theses systems to override the search path for the library.

NixOS users can now have the following devshell/derivation environment when using this package in their projects:

pkgs.mkShell {
   packages = [ ... ];
   shellHook = "... ";
   LLDB_LIB_PATH = "${pkgs.lldb}/lib";
}

[waywardmonkeys]

Thanks for looking into this. We ought to document this stuff in the readme… but that can be done separately.