How to define several include path in Makefile

You need to use -I with each directory. But you can still delimit the directories with whitespace if you use (GNU) make's foreach:

INC=$(DIR1) $(DIR2) ...
INC_PARAMS=$(foreach d, $(INC), -I$d)

You have to prepend every directory with -I:

INC=-I/usr/informix/incl/c++ -I/opt/informix/incl/public