Carview!

Modified the signature for option handlers to accept a pointer to the
matched dropt_option entry. Custom option handlers will need to be
adjusted.
Added a new field to dropt_option to store additional callback data.
This warranted renaming the existing handler_data member. Code that
initialized dropt_option members by name will need to be adjusted.
Added a new dropt_handle_const handler that uses the new callback data
to store predefined values.
Reformatted code and comments.

Fixed a build issue with gcc with optimizations enabled.
Changed dropt_error to be an unsigned int type instead of an enum
type to avoid potential comparison warnings with custom error values.
Fixed dropt_misuse to terminate in debug builds. (I accidentally
disabled termination in dropt 1.1.0.)
Added a Makefile for clang.
Added an INSTALL file with build instructions.
Added a droptxx_example.cpp file as a C++ sample program.
Changed the directory layout a bit.

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