apbs: fix compiler flag passing

pkgs/by-name/ap/apbs/package.nix

@@ -31,12 +31,9 @@ let
       "-DBLAS_LIBRARIES=${blas}/lib"
       "-DBLA_STATIC=OFF"
       "-DBUILD_SUPERLU=OFF"
+      "-DCMAKE_C_FLAGS=-Wno-error=implicit-int"
     ];
 
-    env = lib.optionalAttrs stdenv.cc.isClang {
-      NIX_CFLAGS_COMPILE = "-Wno-error=implicit-int";
-    };
-
     buildInputs = [
       blas
       superlu
@@ -95,12 +92,9 @@ stdenv.mkDerivation (finalAttrs: {
     "-DAPBS_LIBS=mc;maloc"
     "-DCMAKE_MODULE_PATH=${fetk}/share/fetk/cmake;"
     "-DENABLE_TESTS=1"
+    "-DCMAKE_C_FLAGS=-Wno-error=incompatible-pointer-types"
   ];
 
-  env = lib.optionalAttrs stdenv.cc.isClang {
-    NIX_CFLAGS_COMPILE = "-Wno-error=incompatible-function-pointer-types";
-  };
-
   doCheck = true;
 
   meta = with lib; {