treewide: purge explicit usage of gcc6

It's now the default. /cc #19456
This makes a real build simplification, because in our current
bootstrapping+aliases, `gcc6` attribute is not the default compiler
but a derivation *built by* the default compiler.

nix-exec didn't build before this commit already

pkgs/applications/graphics/freepv/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchurl, libjpeg, mesa, freeglut, zlib, cmake, libX11, libxml2, libpng,
-  libXxf86vm, gcc6 }:
+  libXxf86vm }:
 
 stdenv.mkDerivation {
   name = "freepv-0.3.0";
@@ -10,7 +10,7 @@ stdenv.mkDerivation {
   };
 
   buildInputs = [ libjpeg mesa freeglut zlib cmake libX11 libxml2 libpng
-    libXxf86vm gcc6 ];
+    libXxf86vm ];
 
   postPatch = ''
     sed -i -e '/GECKO/d' CMakeLists.txt

pkgs/development/interpreters/nix-exec/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pkgconfig, nix, git, gcc6 }: let
+{ stdenv, fetchurl, pkgconfig, nix, git }: let
   version = "4.1.6";
 in stdenv.mkDerivation {
   name = "nix-exec-${version}";
@@ -8,7 +8,7 @@ in stdenv.mkDerivation {
     sha256 = "0slpsnzzzdkf5d9za7j4kr15jr4mn1k9klfsxibzy47b2bx1vkar";
   };
 
-  buildInputs = [ pkgconfig nix git gcc6 ];
+  buildInputs = [ pkgconfig nix git ];
 
   NIX_CFLAGS_COMPILE = "-std=c++1y";
 

pkgs/development/libraries/audio/suil/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, lib, fetchurl, gtk2, lv2, pkgconfig, python, serd, sord, sratom, gcc6
+{ stdenv, lib, fetchurl, gtk2, lv2, pkgconfig, python, serd, sord, sratom
 , withQt4 ? true, qt4 ? null
 , withQt5 ? false, qt5 ? null }:
 
@@ -16,7 +16,7 @@ stdenv.mkDerivation rec {
     sha256 = "1kji3lhha26qr6xm9j8ic5c40zbrrb5qnwm2qxzmsfxgmrz29wkf";
   };
 
-  buildInputs = [ gtk2 lv2 pkgconfig python serd sord sratom gcc6 ]
+  buildInputs = [ gtk2 lv2 pkgconfig python serd sord sratom ]
     ++ (lib.optionals withQt4 [ qt4 ])
     ++ (lib.optionals withQt5 (with qt5; [ qtbase qttools ]));
 

pkgs/development/libraries/libjson-rpc-cpp/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchFromGitHub, cmake, jsoncpp, argtable, curl, libmicrohttpd
-, doxygen, catch, pkgconfig, git, gcc6
+, doxygen, catch, pkgconfig, git
 }:
 
 stdenv.mkDerivation rec {
@@ -39,7 +39,7 @@ stdenv.mkDerivation rec {
 
     function fixRunPath {
       p=$(patchelf --print-rpath $1)
-      q="$p:${stdenv.lib.makeLibraryPath [ gcc6 jsoncpp argtable libmicrohttpd curl ]}:$out/lib"
+      q="$p:${stdenv.lib.makeLibraryPath [ jsoncpp argtable libmicrohttpd curl ]}:$out/lib"
       patchelf --set-rpath $q $1
     }
 
@@ -53,7 +53,7 @@ stdenv.mkDerivation rec {
     cp -r Install/* $out
   '';
 
-  nativeBuildInputs = [ pkgconfig gcc6 ];
+  nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ cmake jsoncpp argtable curl libmicrohttpd doxygen catch ];
 
   meta = with stdenv.lib; {

pkgs/development/pure-modules/octave/default.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchurl, pkgconfig, pure, octave, gcc6 }:
+{ stdenv, fetchurl, pkgconfig, pure, octave }:
 
 stdenv.mkDerivation rec {
   baseName = "octave";
@@ -10,7 +10,7 @@ stdenv.mkDerivation rec {
     sha256 = "0l1mvmi3rpabzjcrk6p04rdn922mvdm9x67zby3dha5iiccc47q0";
   };
 
-  buildInputs = [ pkgconfig gcc6 ];
+  nativeBuildInputs = [ pkgconfig ];
   propagatedBuildInputs = [ pure octave ];
   makeFlags = "libdir=$(out)/lib prefix=$(out)/";
   setupHook = ../generic-setup-hook.sh;

pkgs/top-level/all-packages.nix

@@ -8424,7 +8424,7 @@ with pkgs;
 
   hwloc = callPackage ../development/libraries/hwloc {};
 
-  hydra = callPackage ../development/tools/misc/hydra { stdenv = overrideCC stdenv gcc6; };
+  hydra = callPackage ../development/tools/misc/hydra { };
 
   hydraAntLogger = callPackage ../development/libraries/java/hydra-ant-logger { };
 
@@ -16373,7 +16373,6 @@ with pkgs;
 
   tdesktop = qt5.callPackage ../applications/networking/instant-messengers/telegram/tdesktop {
     inherit (pythonPackages) gyp;
-    gcc = gcc6;
   };
 
   telegram-cli = callPackage ../applications/networking/instant-messengers/telegram/telegram-cli { };