byte is an Arduino specific, include Arduino.h in your lib.
Now, I'm not sure how good this is going to be for your nightmares, however here is a one liner.
I'll check it through again, but it looks ok.
configuration_UART = ( ( ( bit_parite * 4 ) + ( longueur_donnees == 7 ? 0 : 2 ) + ( bit_arret ? 1 : 0 ) )[ "emgo\x95\x9D\x97\x9F\x85\x8D\x87\x8F" ] ) - 'a';
Questions? 