is_utf8 is a program and a c library to check if a given string is a
valid utf-8 one.

To compile (The program and the lib) :
 $ make

To run :
 $ ./is_utf8 $'\xe9' && echo OK || echo KO

The lib and the program will return 0 (Success) if the sequence is a
valid utf-8 string. Else they will return the byte, starting from one,
where the error was encontred.