Add warnings on undefined functions to client build by default

When building a client, if you mis-spell an API call, there is no warning. If you then try to do the call at runtime, you get an error

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The error doesn't say the symbol name and it doesn't seem to be in the logs either.

Should the build at least warn about using an undefined symbol? The main DR build warns, because it adds --std=gnu99. But the client build options don't add this, perhaps they should? Otherwise, -Wimplicit-function-declaration would work as well.

The runtime error message could print out the symbol name too.