diff --git a/pkgs/by-name/cl/clfft/package.nix b/pkgs/by-name/cl/clfft/package.nix
index b3d55f41e5c0..be470ec69e6b 100644
--- a/pkgs/by-name/cl/clfft/package.nix
+++ b/pkgs/by-name/cl/clfft/package.nix
@@ -46,7 +46,7 @@ stdenv.mkDerivation (finalAttrs: {
   ];
 
   # https://github.com/clMathLibraries/clFFT/issues/237
-  CXXFLAGS = "-std=c++98";
+  env.CXXFLAGS = "-std=c++98";
 
   meta = {
     description = "Library containing FFT functions written in OpenCL";
diff --git a/pkgs/by-name/cr/cryptopp/package.nix b/pkgs/by-name/cr/cryptopp/package.nix
index a57949bea97c..bcc57aa3972a 100644
--- a/pkgs/by-name/cr/cryptopp/package.nix
+++ b/pkgs/by-name/cr/cryptopp/package.nix
@@ -42,7 +42,10 @@ stdenv.mkDerivation rec {
 
   enableParallelBuilding = true;
   hardeningDisable = [ "fortify" ];
-  CXXFLAGS = lib.optionals withOpenMP [ "-fopenmp" ];
+
+  env = lib.optionalAttrs withOpenMP {
+    CXXFLAGS = "-fopenmp";
+  };
 
   doCheck = true;
 
diff --git a/pkgs/by-name/cs/csmith/package.nix b/pkgs/by-name/cs/csmith/package.nix
index 261f5bf1bbf0..80830a36167a 100644
--- a/pkgs/by-name/cs/csmith/package.nix
+++ b/pkgs/by-name/cs/csmith/package.nix
@@ -29,7 +29,7 @@ stdenv.mkDerivation rec {
     SysCPU
   ]);
 
-  CXXFLAGS = "-std=c++98";
+  env.CXXFLAGS = "-std=c++98";
 
   postInstall = ''
     substituteInPlace $out/bin/compiler_test.pl \