diff --git a/pkgs/by-name/_9/_9base/package.nix b/pkgs/by-name/_9/_9base/package.nix
index 983b569062bcb..02aa841117af5 100644
--- a/pkgs/by-name/_9/_9base/package.nix
+++ b/pkgs/by-name/_9/_9base/package.nix
@@ -40,21 +40,17 @@ stdenv.mkDerivation {
   enableParallelBuilding = true;
   strictDeps = true;
   nativeBuildInputs = [ pkg-config ];
-  env.NIX_CFLAGS_COMPILE = toString (
-    [
-      # workaround build failure on -fno-common toolchains like upstream
-      # gcc-10. Otherwise build fails as:
-      #   ld: diffio.o:(.bss+0x16): multiple definition of `bflag'; diffdir.o:(.bss+0x6): first defined here
-      "-fcommon"
-      # hide really common warning that floods the logs:
-      #   warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
-      "-D_DEFAULT_SOURCE"
-    ]
-    ++ lib.optionals stdenv.cc.isClang [
-      # error: call to undeclared function 'p9mbtowc'; ISO C99 and later do not support implicit function declarations
-      "-Wno-error=implicit-function-declaration"
-    ]
-  );
+  env.NIX_CFLAGS_COMPILE = toString ([
+    # workaround build failure on -fno-common toolchains like upstream
+    # gcc-10. Otherwise build fails as:
+    #   ld: diffio.o:(.bss+0x16): multiple definition of `bflag'; diffdir.o:(.bss+0x6): first defined here
+    "-fcommon"
+    # hide really common warning that floods the logs:
+    #   warning: #warning "_BSD_SOURCE and _SVID_SOURCE are deprecated, use _DEFAULT_SOURCE"
+    "-D_DEFAULT_SOURCE"
+    # error: call to undeclared function 'p9mbtowc'; ISO C99 and later do not support implicit function declarations
+    "-Wno-error=implicit-function-declaration"
+  ]);
   env.LDFLAGS = lib.optionalString enableStatic "-static";
   makeFlags = [
     "PREFIX=${placeholder "out"}"