stdenv: start deprecating non-list cmakeFlags
stdenv: start deprecating non-list cmakeFlags

the motivation for this is to simplify stdenv and ease the job of
reviewers due to them needing to tell contributors about the defacto
rule that cmakeFlags should be a list of strings

NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs,NixOS/nixpkgs