src: make and use a config.h file

src: make and use a config.h file

Not every system is the same.

Automatically create a config.h file that contains
"interesting" information about a system that cannot
be determined at compile time via predefined macros.

Provide this file via a CONFIG_H macro to interested
source files that can then `#include CONFIG_H` when
it's defined.

Currently only the framework is provided.  In other
words, the config.h file is created but is always
empty and the proper CONFIG_H macro definition is
passed on to interested source files.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>