gfortran/gcc: simplify & clean up on Darwin

This should eliminate the branched logic for gfortran on Darwin, as well
as preventing accidental inclusion of impure paths in gcc and gfortran
builds.

pkgs/development/compilers/gcc/5/default.nix

@@ -76,7 +76,8 @@ let version = "5.4.0";
       ++ optional langAda ../gnat-cflags.patch
       ++ optional langFortran ../gfortran-driving.patch
 
-      # This could be applied unconditionally but I don't want to cause a full Linux rebuild.
+      # This could be applied unconditionally but I don't want to cause a full
+      # Linux rebuild.
       ++ optional stdenv.cc.isClang ./libcxx38-and-above.patch;
 
     javaEcj = fetchurl {
@@ -234,6 +235,13 @@ stdenv.mkDerivation ({
 
   libc_dev = stdenv.cc.libc_dev;
 
+  # This should kill all the stdinc frameworks that gcc and friends like to
+  # insert into default search paths.
+  prePatch = if stdenv.isDarwin then ''
+    substituteInPlace gcc/config/darwin-c.c \
+      --replace 'if (stdinc)' 'if (0)'
+  '' else null;
+
   postPatch =
     if (stdenv.isGNU
         || (libcCross != null                  # e.g., building `gcc.crossDrv'

pkgs/top-level/all-packages.nix

@@ -5058,10 +5058,7 @@ with pkgs;
     isl = if !stdenv.isDarwin then isl_0_14 else null;
   }));
 
-  gfortran = if !stdenv.isDarwin then gfortran5
-             else callPackage ../development/compilers/gcc/gfortran-darwin.nix {
-    inherit (darwin) Libsystem;
-  };
+  gfortran = gfortran5;
 
   gfortran48 = wrapCC (gcc48.cc.override {
     name = "gfortran";