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

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