treewide: avoid fatal warnings from gcc7

pkgs/development/libraries/libwps/default.nix

@@ -12,6 +12,8 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ boost librevenge zlib ];
 
+  NIX_CFLAGS_COMPILE = [ "-Wno-error=implicit-fallthrough" ]; # newly detected by gcc-7
+
   meta = with stdenv.lib; {
     homepage = http://libwps.sourceforge.net/;
     description = "Microsoft Works document format import filter library";