gcc6: move dependencies to common/dependencies.nix

the (vc4 redox) flex conditional was missing in all other gcc version
because gcc6 was the default at the time it was added

that is one of the problems common/dependencies.nix fixes

pkgs/development/compilers/gcc/6/default.nix

@@ -193,38 +193,40 @@ stdenv.mkDerivation ({
   inherit noSysDirs staticCompiler langJava crossStageStatic
     libcCross crossMingw;
 
-  depsBuildBuild = [ buildPackages.stdenv.cc ];
-  nativeBuildInputs = [ texinfo which gettext ]
-    ++ (optional (perl != null) perl)
-    ++ (optional javaAwtGtk pkg-config)
-    ++ (optional (with stdenv.targetPlatform; isVc4 || isRedox) flex)
-    ++ (optional langAda gnatboot)
-    # The builder relies on GNU sed (for instance, Darwin's `sed' fails with
-    # "-i may not be used with stdin"), and `stdenvNative' doesn't provide it.
-    ++ (optional buildPlatform.isDarwin gnused)
+  inherit (import ../common/dependencies.nix {
+    inherit
+      lib
+      stdenv
+      buildPackages
+      targetPackages
+      crossStageStatic
+      threadsCross
+      version
+      langAda
+      gnatboot
+      flex
+      langJava
+      javaAwtGtk
+      texinfo
+      which
+      gettext
+      pkg-config
+      gnused
+      patchelf
+      gmp
+      mpfr
+      libmpc
+      isl
+      zlib
+      boehmgc
+      zip
+      unzip
+      gtk2
+      libart_lgpl
+      perl
+      xlibs
     ;
-
-  # For building runtime libs
-  depsBuildTarget =
-    (
-      if hostPlatform == buildPlatform then [
-        targetPackages.stdenv.cc.bintools # newly-built gcc will be used
-      ] else assert targetPlatform == hostPlatform; [ # build != host == target
-        stdenv.cc
-      ]
-    )
-    ++ optional targetPlatform.isLinux patchelf;
-
-  buildInputs = [
-    gmp mpfr libmpc
-    targetPackages.stdenv.cc.bintools # For linking code at run-time
-  ] ++ (optional (isl != null) isl)
-    ++ (optional (zlib != null) zlib)
-    ++ (optionals langJava [ boehmgc zip unzip ])
-    ++ (optionals javaAwtGtk ([ gtk2 libart_lgpl ] ++ xlibs))
-    ;
-
-  depsTargetTarget = optional (!crossStageStatic && threadsCross != {}) threadsCross.package;
+  }) depsBuildBuild nativeBuildInputs depsBuildTarget buildInputs depsTargetTarget;
 
   NIX_LDFLAGS = lib.optionalString  hostPlatform.isSunOS "-lm -ldl";
 

pkgs/development/compilers/gcc/common/dependencies.nix

@@ -15,6 +15,8 @@
 , cloog ? null
 , isl ? null
 , zlib ? null
+, gnatboot ? null
+, flex ? null
 , boehmgc
 , zip
 , unzip
@@ -22,8 +24,9 @@
 , libart_lgpl
 , perl ? null
 , xlibs ? null
-, langJava ? null
-, javaAwtGtk ? null
+, langJava ? false
+, javaAwtGtk ? false
+, langAda ? false
 , crossStageStatic ? null
 , threadsCross ? null
 }:
@@ -44,6 +47,8 @@ in
   ]
   ++ optionals (perl != null) [ perl ]
   ++ optionals javaAwtGtk [ pkg-config ]
+  ++ optionals (with stdenv.targetPlatform; isVc4 || isRedox) [ flex ]
+  ++ optionals langAda [ gnatboot ]
   # The builder relies on GNU sed (for instance, Darwin's `sed' fails with
   # "-i may not be used with stdin"), and `stdenvNative' doesn't provide it.
   ++ optionals buildPlatform.isDarwin [ gnused ]