autoconf: Detect boost, require version, set CXXFLAGS.
autoconf: Detect boost, require version, set CXXFLAGS.

This turns previous compiler errors complaining about missing files
into proper ./configure time errors telling the user which version
of boost is required.

NixOS/nix,ehmry/nix,dtzWill/nix,rycee/nix,pikajude/nix,pikajude/nix,NixOS/nix,rycee/nix,rycee/nix,ehmry/nix,dtzWill/nix,NixOS/nix,ehmry/nix,rycee/nix,rycee/nix,dtzWill/nix,NixOS/nix,ehmry/nix,NixOS/nix,dtzWill/nix,dtzWill/nix,pikajude/nix,ehmry/nix,ehmry/nix