Convert a character to ASCII upper case.
If the character is not an ASCII lower case letter, it is returned unchanged.
Unlike the standard C library `toupper()` function, this only recognizes standard ASCII letters and ignores the locale, returning all non-ASCII characters unchanged, even if they are upper case letters in a particular character set. Also unlike the standard library function, this takes and returns a char, not an int, so don't call it on `EOF` but no need to worry about casting to `guchar` before passing a possibly non-ASCII character in.
| c |
any character |
|
the result of the conversion |