guile-reader: fix build with clang 16

pkgs/development/guile-modules/guile-reader/default.nix

···
     libffi
   ];
 
-  env.GUILE_SITE = "${guile-lib}/${guile.siteDir}";
+  env = {
+    GUILE_SITE = "${guile-lib}/${guile.siteDir}";
+  } // lib.optionalAttrs stdenv.cc.isClang {
+    NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration";
+  };
 
   configureFlags = [ "--with-guilemoduledir=$(out)/${guile.siteDir}" ];