ax_declspec.m4: Make __declspec(dllexport) test fail for Haiku OS
as its GCC does not support the attribute.
configure.ac: Check for network related functions in libnetwork to support Haiku OS
and in libnet to (possibly) support BeOS.

configure: Regenerate.
README: Add a note about Haiku OS support.
README: "Haiku", not "Haiku OS".