gcc: 7.3 -> 7.4

https://gcc.gnu.org/gcc-7/changes.html

pkgs/development/compilers/gcc/7/default.nix

@@ -37,7 +37,7 @@ assert langGo -> langCC;
 with stdenv.lib;
 with builtins;
 
-let version = "7.3.0";
+let version = "7.4.0";
 
     inherit (stdenv) buildPlatform hostPlatform targetPlatform;
 
@@ -138,7 +138,7 @@ stdenv.mkDerivation ({
 
   src = fetchurl {
     url = "mirror://gcc/releases/gcc-${version}/gcc-${version}.tar.xz";
-    sha256 = "0p71bij6bfhzyrs8676a8jmpjsfz392s2rg862sdnsk30jpacb43";
+    sha256 = "0lgy170b0pp60j9cczqkmaqyjjb584vfamj4c30swd7k0j6y5pgd";
   };
 
   inherit patches;