dropt 1.1.1

• 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.