This adds the mal_context_config structure and changes the
mal_context_init() API to take a pointer to a config object.

Thanks for that one, Clang!

  * Fix link errors with Clang
  * Add support for VC++ 6. Note that WASAPI and DirectSound are currently
    disabled for this compiler.

  * Generally improve latency.
  * Get MMAP mode working with playback devices.
    * This can be disabled with the alsa.noMMap config.
  * No longer default to using "plughw" devices and instead just use "hw"
    devices where appropriate.
    * This can be disabled with the alsa.preferPlugHW config.

This removes the need to the -lasound linker option.

This should remove the need to link to libpthread.