This is probably a West African form of
Ahmad. I found one site in French that stated that directly. Then the following site on Gambian names:
http://resourcepage.gambia.dk/names.htm
says that Amadou was "one of the names of
Muhammad", and
Digest of Muslim Names by Fatimah
Suzanne Al Jafari,
A Dictionary of Muslim Names by Salahuddin
Ahmed, and
The Complete Book of Muslim & Parsi Names by Gandhi &
Husain all agree that
Ahmed is one of the names of
Muhammad. So it seems probable to me that Amadou is derived from
Ahmed or
Ahmad.