figlet: ignore implicit-function-declaration; fix build

figlet.c uses getopt from getopt.c with no function declaration.

pkgs/tools/misc/figlet/default.nix

···
     })
   ];
 
+  env = lib.optionalAttrs stdenv.cc.isClang {
+    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration";
+  };
+
   makeFlags = [ "prefix=$(out)" "CC:=$(CC)" "LD:=$(CC)" ];
 
   postInstall = "cp -ar ${contributed}/* $out/share/figlet/";