Debugging FB

For debugging and development it's a good idea to build the compiler with 
-g and -exx to enable assertions, plus array boundary and null-pointer 
checks. For the rtlib/gfxlib2 code, -DDEBUG enables the assertions. Just 
update config.mk and (re)build. Example config.mk settings:

   FBFLAGS := -g -exx
   CFLAGS := -g -O0 -DDEBUG

Running fbc inside gdb typically looks like this:

   gdb --args fbc foo.bas

Running fbc inside valgrind typically looks like this:

   valgrind fbc foo.bas

Also note that fbc can be tested right from inside the build tree, without 
having to be "installed" somewhere else, which also is a great debugging 
and development help.

