add '--enable-debug' configure option to build with EMIT_ASN_DEBUG

... which in turn causes all the ASN_DEBUG() to be turned into
fprintf(stderr, ...) statements, once the user application decides
to set 'asn_debug = 1' somewhere in its code.

The next step would be to make _ASN_DECODE_FAILED / _ASN_ENCODE_FAILED
no longer depend on ASN_DEBUG (which it currently does)
4 files changed