gcc: add --with-native-system-header-dir for clang

gcc needs to be able find system headers. Without this, gcc fails to build because
/usr/include is not available.

Note: stdenv.libc should be available for all stdenv's, I think.

pkgs/development/compilers/gcc/4.8/default.nix

@@ -357,8 +357,8 @@ stdenv.mkDerivation ({
         )
       )
     }
-    ${if (stdenv ? glibc && cross == null)
-      then " --with-native-system-header-dir=${stdenv.glibc.dev}/include"
+    ${if cross == null
+      then " --with-native-system-header-dir=${getDev stdenv.libc}/include"
       else ""}
     ${if langAda then " --enable-libada" else ""}
     ${if cross == null && stdenv.isi686 then "--with-arch=i686" else ""}

pkgs/development/compilers/gcc/4.9/default.nix

@@ -364,8 +364,8 @@ stdenv.mkDerivation ({
         )
       )
     }
-    ${if (stdenv ? glibc && cross == null)
-      then " --with-native-system-header-dir=${stdenv.glibc.dev}/include"
+    ${if cross == null
+      then " --with-native-system-header-dir=${getDev stdenv.libc}/include"
       else ""}
     ${if langAda then " --enable-libada" else ""}
     ${if cross == null && stdenv.isi686 then "--with-arch=i686" else ""}

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

@@ -362,8 +362,8 @@ stdenv.mkDerivation ({
         )
       )
     }
-    ${if (stdenv ? glibc && cross == null)
-      then " --with-native-system-header-dir=${stdenv.glibc.dev}/include"
+    ${if cross == null
+      then " --with-native-system-header-dir=${getDev stdenv.libc}/include"
       else ""}
     ${if langAda then " --enable-libada" else ""}
     ${if cross == null && stdenv.isi686 then "--with-arch=i686" else ""}

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

@@ -360,8 +360,8 @@ stdenv.mkDerivation ({
         )
       )
     }
-    ${if (stdenv ? glibc && cross == null)
-      then " --with-native-system-header-dir=${stdenv.glibc.dev}/include"
+    ${if cross == null
+      then " --with-native-system-header-dir=${getDev stdenv.libc}/include"
       else ""}
     ${if langAda then " --enable-libada" else ""}
     ${if cross == null && stdenv.isi686 then "--with-arch=i686" else ""}