json-c*: fixup build with gcc-7 (ignore warning)

pkgs/development/libraries/json-c/0.11.nix

@@ -6,6 +6,9 @@ stdenv.mkDerivation rec {
     url    = "https://github.com/json-c/json-c/archive/json-c-0.11-20130402.tar.gz";
     sha256 = "1m8fy7lbahv1r7yqbhw4pl057sxmmgjihm1fsvc3h66710s2i24r";
   };
+
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=implicit-fallthrough" ];
+
   meta = with stdenv.lib; {
     description = "A JSON implementation in C";
     homepage    = https://github.com/json-c/json-c/wiki;

pkgs/development/libraries/json-c/default.nix

@@ -11,6 +11,9 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ autoreconfHook ]; # won't configure without it, no idea why
 
+  # probably OK after update: https://github.com/json-c/json-c/pull/365
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=implicit-fallthrough" ];
+
   # compatibility hack (for mypaint at least)
   postInstall = ''
     ln -s json-c.pc "$dev/lib/pkgconfig/json.pc"