Add support for building AIX shared libraries. (It's complicated - we

Add support for building AIX shared libraries.  (It's complicated - we
build "traditional" AIX shared libraries, which are archive libraries
containing shared objects, so, on AIX, we build the archive library, for
the benefit of "build libpcap and tcpdump in subdirectories of the same
directory, with tcpdump linked statically with that libpcap", but don't
install it.  Newer versions of AIX, at least, also support "standard"
.so-style UN*X shared libraries, but we stick with
<Tevye>Tradition!</Tevye>.)

Fix an AIX build problem that shows up when building with GCC.

Clean up HP-UX stuff.