Why doesn't g++ use the -I option?

You need to put -I$(INCDIR) into CPPFLAGS, not LDFLAGS. Then it will be picked up by the built-in rule for compilation of individual object files.

You should also rename CFLAGS to CXXFLAGS and CC to CXX. CC and CFLAGS are for C source files, not C++ source files.

You should not have $(EJECUTABLE) depend on clean, and you should not execute rm -f $(MODULOS) after the link command. Those things defeat the purpose of a Makefile, which is to recompile only what is necessary, not the whole program every single time.

The problem isn't in the compiler but in your Makefile: you want to set up the include directory path in the appropriate flags (I typically use CPPFLAGS but I typically also have my own rules which explicitly reference the flags I'm using). The LDFLAGS are definitely only passed to the linking stage of the build.