c-strcaseeq module contains an optimized case-insensitive
string comparison function operating on single-byte character strings, that
operate as if the locale encoding was ASCII.
(The "C" locale on many systems has the locale encoding "ASCII".)
The functions is actually implemented as a macro:
extern int STRCASEEQ (const char *s1, const char *s2, int s20, int s21, int s22, int s23, int s24, int s25, int s26, int s27, int s28);
s2 should be a short literal ASCII string, and s20, s21, ... the individual characters of s2.
For case conversion here, only ASCII characters are considered to be upper case or lower case.