python3Packages.primer3: fix build on darwin

(the build requires gcc)

pkgs/development/python-modules/primer3/default.nix

@@ -1,7 +1,9 @@
 { lib
+, stdenv
 , buildPythonPackage
 , fetchFromGitHub
 , cython
+, gcc
 , click
 }:
 
@@ -16,7 +18,8 @@ buildPythonPackage rec {
     sha256 = "1glybwp9w2m1ydvaphr41gj31d8fvlh40s35galfbjqa563si72g";
   };
 
-  nativeBuildInputs = [ cython ];
+  nativeBuildInputs = [ cython ]
+    ++ lib.optionals stdenv.isDarwin [ gcc ];
 
   # pytestCheckHook leads to a circular import issue
   checkInputs = [ click ];