libev: Fix compilation with clang

pkgs/development/libraries/libev/default.nix

@@ -7,6 +7,11 @@ stdenv.mkDerivation rec {
     url = "http://dist.schmorp.de/libev/${name}.tar.gz";
     sha256 = "1jyw7qbl0spxqa0dccj9x1jsw7cj7szff43cq4acmklnra4mzz48";
   };
+
+  # Version 4.19 is not valid C11 (which Clang default to)
+  # Check if this is still necessary on upgrade
+  NIX_CFLAGS_COMPILE = if stdenv.cc.isClang then "-std=c99" else null;
+
   meta = {
     description = "A high-performance event loop/event model with lots of features";
     maintainers = [ stdenv.lib.maintainers.raskin ];