Override automake's DEFAULT_INCLUDES.

Override automake's DEFAULT_INCLUDES.

Automake adds all kinds of subdirectories to -I, the list of directories
to be searched for header files. However, CLN's header files are not
supposed to be included without the cln/ subdirectory, so adding
-Iinclude/cln can lead to conflicts with other header files. (The
problem at hand was a conflict between CLN's string.h and the C
library's string.h).