fox: mark broken on darwin

Undefined symbols for architecture x86_64:
"_MPCreateSemaphore", referenced from:
FX::FXSemaphore::FXSemaphore(int) in FXThread.o
FX::FXSemaphore::FXSemaphore(int) in FXThread.o
"_MPDeleteSemaphore", referenced from:
FX::FXSemaphore::~FXSemaphore() in FXThread.o
FX::FXSemaphore::~FXSemaphore() in FXThread.o
"_MPSignalSemaphore", referenced from:
FX::FXSemaphore::post() in FXThread.o
"_MPWaitOnSemaphore", referenced from:
FX::FXSemaphore::wait() in FXThread.o
FX::FXSemaphore::trywait() in FXThread.o
ld: symbol(s) not found for architecture x86_64

/cc ZHF #45961

pkgs/development/libraries/fox/default.nix

···
     homepage = http://fox-toolkit.org;
     license = licenses.lgpl3;
     maintainers = [];
+    broken = stdenv.isDarwin;
     platforms = platforms.all;
   };
 }