Get rid of all with { inherit... } and just used let inherit...

The old forms presumably predates, or were made in ignorance of,
`let inherit`. This way is better style as the scoping as more lexical,
something which Nix can (or might already!) take advantage of.

pkgs/development/libraries/gmp/5.1.x.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, m4, cxx ? true, withStatic ? true }:
 
-with { inherit (stdenv.lib) optional optionalString; };
+let inherit (stdenv.lib) optional optionalString; in
 
 let self = stdenv.mkDerivation rec {
   name = "gmp-5.1.3";

pkgs/development/libraries/gmp/6.x.nix

@@ -1,6 +1,6 @@
 { stdenv, fetchurl, m4, cxx ? true, withStatic ? false }:
 
-with { inherit (stdenv.lib) optional optionalString; };
+let inherit (stdenv.lib) optional optionalString; in
 
 let self = stdenv.mkDerivation rec {
   name = "gmp-6.1.1";