Makefile: add support for ABC_USE_STDINT_H

Use C99 header stdint.h to determine platform-dependent types and macros

README: minor updates

point to github instead of bitbucket

CMake: Ensure abc executable depends on libabc

This ensures that libabc is built only once.

Previously the code for libabc was being built twice: once for the
abc executable target, and again for the libabc target.

If ABC_HAVE_STDINT_H is defined, standard C99 headers will be used
to define all of the platform-dependent types required. arch_flags
will also no longer be required. This new define is optional and
must be manually enabled by setting ARCHFLAGS.