This is useful because MA_ASSERT is only defined in the implementation
section of miniaudio.h which can cause issues when people copy/paste
the code and use it in a file that does not have visibility of the
implementation.

Note that there are still more references to implementation-defined
macros, but these have been moved to the public section in the dev-0.12
branch so I'm not bothering to change those just yet.

Public issue https://github.com/mackron/miniaudio/issues/787