Using checks of the existence of HEADER_{foo}_H in other header files

was a really bad idea.  For example, the following:

	#include <x509.h>
	#include <bio.h>
	#include <asn1.h>

would make sure that things like ASN1_UTCTIME_print() wasn't defined
unless you moved the inclusion of bio.h to above the inclusion of
x509.h.  The reason is that x509.h includes asn1.h, and the
declaration of ASN1_UTCTIME_print() depended on the definition of
HEADER_BIO_H.  That's what I call an obscure bug.

Instead, this change makes sure that whatever header files are needed
for the correct process of one header file are included automagically,
and that the definitions of, for example, BIO-related things are
dependent on the absence of the NO_{foo} macros.  This is also
consistent with the way parts of OpenSSL can be excluded at will.