Use unique variables for cc65 toolchain.

Originally I used the usual variables (like $(CC) and $(CFLAGS) ) but after
all this doesn't make sense as any predefined values and/or user defined
settings can only be wrong.
Added support for 'prefix'.

If the variable 'prefix' is defined then the builtin search paths are set to
$(prefix)/lib/cc65/... allowing to build binaries intended for installation.

Note that the library build still works with these binaries as it generally
overrides the builtin search paths by setting the CC65_HOME env var.
Use symbolic names for all CIO commands.
remove TABs
Merge pull request #14 from groessler/something_to_pull

Use symbolic names for all CIO commands.
Avoid GCC "unconst" warning.
Have __APPLE2ENH__ imply __APPLE2__ and __ATARIXL__ imply __ATARI__.
Avoid macro redefinition of CH_DEL for apple2enh.

I'm wondering why MS VC++ didn't complain about this...
Simply define CH_DEL exactly then when there none yet.
Fix writing one byte beyond screen buffer.
Merge pull request #15 from groessler/something_to_pull

Fix writing one byte beyond screen buffer.
Fixed comment.
Fixed comment.
move findfreeiocb function into its own object file
Merge pull request #16 from groessler/something_to_pull

move findfreeiocb function into its own object file
No delimiter comment between includes and first segment.
Have __PLUS4__ imply __C16__.
Made EXTZP segment option for modules.

This allows to remove the "add empty EXTZP segement to avoid warnings"
approach.
Merge branch 'master' into lightpen
Look for NULL pointer more efficiently.