motif: fix build with GCC 15

Getting this package to support C23 would be nontrivial; see
https://gitlab.alpinelinux.org/alpine/aports/-/commit/c884c1f3642e96d7de6d905b828170db5bdd2c56

We also need to enable GNU extensions for `caddr_t`.

pkgs/by-name/mo/motif/package.nix

@@ -122,12 +122,15 @@ stdenv.mkDerivation rec {
     "ac_cv_func_setpgrp_void=${lib.boolToYesNo (!stdenv.hostPlatform.isBSD)}"
   ];
 
-  env = lib.optionalAttrs stdenv.cc.isClang {
-    NIX_CFLAGS_COMPILE = toString [
+  env.NIX_CFLAGS_COMPILE = toString (
+    [
+      "-std=gnu17"
+    ]
+    ++ lib.optionals stdenv.cc.isClang [
       "-Wno-error=implicit-function-declaration"
       "-Wno-error=incompatible-function-pointer-types"
-    ];
-  };
+    ]
+  );
 
   enableParallelBuilding = true;