pcalc: fix build with GCC 15 (#477450)

pkgs/by-name/pc/pcalc/package.nix

@@ -17,6 +17,11 @@ stdenv.mkDerivation {
     sha256 = "sha256-m4xdsEJGKxLgp/d5ipxQ+cKG3z7rlvpPL6hELnDu6Hk=";
   };
 
+  # Since C23, coercing functions with different parameter lists to a function pointer with no
+  # parameter specified triggers a hard error: `symbol.c:92:22: error: initialization of
+  # 'int (*)(void)' from incompatible pointer type 'int (*)(double)' [-Wincompatible-pointer-types]`
+  env.NIX_CFLAGS_COMPILE = "-std=gnu17";
+
   makeFlags = [ "DESTDIR= BINDIR=$(out)/bin" ];
   nativeBuildInputs = [
     bison