Two questions about the second (and blocking) problem:
 1- can you return here the value of FLEXIBLE_ARRAY_MEMBER
  from config.h? (it should be set to 1 for an old compiler,
  to nothing for a C 99 compiler)

 2- replace in lib/isc/ht.c the [] after key by
  [FLEXIBLE_ARRAY_MEMBER].

It should compile with this (if there is no other problem).

I am still interested by your compiler version.
According to Gnu docs the deprecated attribute is
supported by gcc 3.1.1 and perhaps older (I found
only 3.0.4 and 2.95.3 docs, both without this atttribute).