Add support for enabling only specific backends at compile time.

This enables the ability to explicitly enable only the backends a
program is interested in. Essentially it's the reverse of the pre-
existing method whereby instead of disabling backends, all backends are
disabled by default, and then specific backends are enabled. Example:

    #define MA_ENABLE_ONLY_SPECIFIC_BACKENDS
    #define MA_ENABLE_WASAPI /* Only care about WASAPI on Windows. */
    #define MA_ENABLE_ALSA   /* Only care about ALSA on Linux. */

Note that even if MA_ENABLE_* is used, the backend will still only be
enabled if the compilation environment and target platform actually
supports it. You can therefore use the MA_ENABLE_* options without
needing to worry about platform detection.

Public issue https://github.com/mackron/miniaudio/issues/260