Use ASCII version of strcasecmp() in library code

strcasecmp() has side effects in some locales. What we really need is c_strcasecmp() from Gnulib for comparing ASCII strings.

Fixes #570

Checklist

• Code modified for feature

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code