How to define several include path in Makefile

New to C++; Basic understanding of includes, libraries and the compile process. Did a few simple makefiles yet.

My current project involves using an informix DB api and i need to include header files in more than one nonstandard dirs. How to write that ? Havent found anything on the net, probably because i did not use good search terms

This is one way what i tried (not working). Just to show the makefile

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

default:    main

main:   test.cpp
        gcc -Wall $(LIB) $(INC) -c test.cpp
        #gcc -Wall $(LIB) $(INC) -I/opt/informix/incl/public -c test.cpp

clean:
        rm -r test.o make.out

You have to prepend every directory with -I:

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

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)

Make's substitutions feature is nice and helped me to write

%.i: src/%.c $(INCLUDE)
        gcc -E $(CPPFLAGS) $(INCLUDE:%=-I %) $< > $@

You might find this useful, because it asks make to check for changes in include folders too