(tl;dr: this fixes a bug in avrgcc that's present in master and in the current release)

gcc provides wrappers for binutils' ar, nm and ranlib
executables, which must be used instead when using link-time
optimisation. See also:
http://manpages.ubuntu.com/manpages/zesty/man1/aarch64-linux-gnu-gcc-ar-5.1.html

The upstream version of avr-gcc-ar searches in paths passed to
the configure script for the avr-ar binary that it wraps, falling
back to searching PATH instead. Thus currently avr-gcc-ar works on
Nix, but only if avrbinutils is already in the environment.

This change bakes the path to avr-ar into avr-gcc-ar, since its path
is known at compile time. It also no longer searches PATH, meaning the
user's local environment won't override this path.

Note that avr-gcc-nm and avr-gcc-ranlib are compiled from the same
source file as avr-gcc-ar, just with different compiler flags.

Testing on master (without avrbinutils in the environment):

$ nix-build -A avrgcc
$ result/bin/avr-gcc-ar --version
result/bin/avr-gcc-ar: Cannot find binary 'avr-ar'

Testing on branch with this fix:

$ nix-build -A avrgcc
$ result/bin/avr-gcc-ar --version
GNU ar (GNU Binutils) 2.26.20160125
...

Motivation for this change

avr-gcc-ar shouldn't require any particular package to be present in the user's environment when it is run.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

cc package maintainer @mguentner