Bug#717014: gnat-4.6: on some archs, a library using Elementary_Functions needs -lm

[Nicolas Boulenguez <nicolas@debian.org>]

Package: gnat-4.6
Followup-For: Bug #717014

For the record, here are two sensible work-arounds:
* in linker options:
  -Wl,--as-needed -lm -Wl,--no-as-needed
* in a GNAT project:
  case External ("NEEDS_LM") is
      when "yes" =>
         for Library_Options use ("-lm");
      when "no" =>
         null;
  end case;