The first set of variables controls the directories that are recognised in the makefile. They should
not be set in the Makefile.fpc file, but can be specified on the commandline.
     
- 
INCDIR 
- this is a list of directories, separated by spaces, that will be added as include
     directories to the compiler command-line. Each directory in the list is prepended with
     -I and added to the compiler options.
     
- 
UNITDIR 
- this is a list of directories, separated by spaces, that will be added as unit search
     directories to the compiler command-line. Each directory in the list is prepended with
     -Fu and added to the compiler options.
     
- 
LIBDIR 
- is a list of library paths, separated by spaces. Each directory in the list is prepended
     with -Fl and added to the compiler options.
     
- 
OBJDIR 
- is a list of object file directories, separated by spaces, that is added to the object
     files path, i.e. Each directory in the list is prepended with -Fo.