OK, danke. Ich glaube ich habe die Stelle gefunden.
Code:
# List any extra directories to look for include files here. 
#     Each directory must be seperated by a space. 
#     Use forward slashes for directory separators. 
#     For a directory that has spaces, enclose it in quotes. 
EXTRAINCDIRS = $(AT90_LIB_PATH) $(AT90_LIB_PATH_OTHERS)
Danke für den entscheidenden Hinweis!

MfG
Filou