This encloses all user configuration options with preprocessor
ifndef ... endif guards, so that the configuration can be applied
already from the Makefile (for example with gcc -D_WIZCHIP_=5100 or
when using CMake: add_definitions(-D_WIZCHIP_=5100)).
The advantage of this change is that the ioLibrary can then simply be
included as a git submodule from the official upstream repository into
the own project without having to carry along a patch of
wizchip_conf.h.
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