configure.ac: fix use of unread LIBS variable
configure.ac: fix use of unread LIBS variable

This fixes both the SunOS/Solaris check, and the libatomic check, which
reference $LIBS, which has not been used since automake was stripped
out of the code.

rycee/nix,rycee/nix,NixOS/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix,NixOS/nix,rycee/nix