[build] A simple way to append to include path -

currently one needs to do `CFLAGS += -I../../../myproject/include`
if they add more directories to the include path, and this only
appears to work from toplevel Makefile (e.g. from the examples/).
If one wants to bundle a library with their app, they can use
`PROJECT_LIBRARIES`, BUT they cannot really ammend CFLAGS from
the app's makefile. So this change is mainly intending to solve
that particular problem.