diff --git a/pkgs/development/compilers/gcc/7/default.nix b/pkgs/development/compilers/gcc/7/default.nix
index bb8a3638b2d..e2c686b7e7e 100644
--- a/pkgs/development/compilers/gcc/7/default.nix
+++ b/pkgs/development/compilers/gcc/7/default.nix
@@ -217,9 +217,7 @@ stdenv.mkDerivation ({
     ++ (optional hostPlatform.isDarwin targetPackages.stdenv.cc.bintools)
     ;
 
-  # TODO: Use optionalString with next rebuild.
-  ${if (stdenv.cc.isClang && langFortran) then "NIX_CFLAGS_COMPILE" else null} = "-Wno-unused-command-line-argument";
-
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString (stdenv.cc.isClang && langFortran) "-Wno-unused-command-line-argument";
   NIX_LDFLAGS = stdenv.lib.optionalString  hostPlatform.isSunOS "-lm -ldl";
 
   preConfigure = stdenv.lib.optionalString (hostPlatform.isSunOS && hostPlatform.is64bit) ''