Separate out function and function pointer attributes, as not all
versions of all compilers support both.
Have macros related to function attributes auto-generated by
autoheader. As a result, rename config.h.in to unconfig.h, to make it
more obvious that it is really intended to be included from some C
programs.
Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>