This cleans up libavrdude.h a bit and installs libavrdude-avrintel.h:

• libavrdude.h will create an error if LIBAVRDUDE_INCLUDE_INTERNAL_HEADERS is defined
• add comment on top of libavrdude-avrintel.h about not directly #includeing it
• install libavrdude-avrintel.h alongside libavrdude.h (both cmake and autotools)
• use proper C99 and later definitions for a C function without parameters: rettype funcname(void);
• have the libavrdude.h/libavrdude-avrintel.h combination #include all required libc headers declaring the types used in them (add stddef.h for size_t)

Test programs for testing language compatibility will come later when I figure out how to build them in cmake. A program like the following should build with all compilers and all supported language standards:

#include <libavrdude.h>

int main(void) {
  return 0;
}