Add support for some non-alphanumeric variable names in the math expr evaluator

[GitHub issues - opened <github@xxxxxxxxxxxxxxxx>]

alsa-project/alsa-lib issue #197 was opened from ranj063:

The math expression evaluator expects variable names to only be alpha-numeric. So if I have something like "[$DMIC_CH_COUNT + 2]", I get the error:

Issue URL     : https://github.com/alsa-project/alsa-lib/issues/197
Repository URL: https://github.com/alsa-project/alsa-lib