Fix compilation on OS X

Linking on OS X, I was getting this error:
```
Undefined symbols for architecture x86_64:
  "_alloca", referenced from:
      _array_get_bang in libpd-osx.a(x_array.o)
      _list_split_anything in libpd-osx.a(x_list.o)
      _list_prepend_list in libpd-osx.a(x_list.o)
      _list_prepend_anything in libpd-osx.a(x_list.o)
      _list_append_list in libpd-osx.a(x_list.o)
      _list_append_anything in libpd-osx.a(x_list.o)
      _netreceive_readbin in libpd-osx.a(x_net.o)
      ...
```
Adding `-DHAVE_ALLOCA_H` to the other C flags fixed the build. This is on OS X 10.10, Xcode 6.1.