Return length of prefix in string of chars NOT in set.
──────────────────────────────────────────────────────────────────────────────

 Syntax

        GT_strcspn(<cString>, <cSet>) --> nLength

 Arguments:

      <cString> - The string to find the prefix in
      <cSet>    - The set of characters

 Returns:

      nLength   - The length of a string upto a character in the set

 Description:

      Return the number of characters in the leading segment of a
      string that consists solely of characters NOT in the set.

 Examples:


      ? GT_strcspn("this is a test", "as ")      // prints 3
      ? GT_strcspn("this is a test", "elnjpq")   // prints 11


 Source: STRCSPN.C

 Author:  Andy M Leighton