getaddrinfo is fully thread safe on solaris versions which
implement the function even when h_errno is not a macro.

The h_errno macro test now only done on systems for which there
is no hard coded knowledge about getaddrinfo's thread safeness.

Refactor how preprocessor symbol _THREAD_SAFE definition is done.

knowledge says the contrary or h_errno is not defined.

- Constantine Sapuntzakis reported that Darwin 6.0 a.k.a. MAC OS X 10.2
  and newer have a threadsafe getaddrinfo.

- Fix Dragonfly BSD triplet detection.

- In case the hard-coded knowledge says that getaddrinfo is threadsafe,
  an additional check is done to verify that h_errno is also defined.
  If h_errno isn't defined, we finally assume that it isn't threadsafe.
  Jamie Lokier provided the inspiration for this extra check.

Add some comments to better understand what the regex's pretend to achieve.

Undo old temporary change once used for testing purposes

Symbol hiding configure options renamed to the hopefully less ambiguous
--enable-symbol-hiding and --disable-symbol-hiding as well as related
macro names and some internal variables used for them.

Related configuration file preprocessor symbols named to
CARES_SYMBOL_HIDING and CARES_SYMBOL_SCOPE_EXTERN.

based on the 'visibility' attribute for GNUC and __global for Sun
compilers, taking also in account __declspec function decoration
for Win32 and Symbian DLL's.

Introducing configure options --enable-hidden-symbols and
--disable-hidden-symbols following libcurl's naming.

is only expanded and included once in the configure script.

due to autoconf 2.64 m4_require'ing it in _AS_SHELL_SANITIZE indirectly through
_AS_PATH_WALK.

make errno available as a preprocessor macro.

renamed generated config.h to ares_config.h in order to avoid clashes when libcurl is used with other projects which also have a config.h.

Refactor how libraries are checked for connect() function,
and check for connect() as it is done for other functions.

check for socket() and closesocket() as it is done for other functions

systems which support external, undefined, symbols in shared libraries.

Moved *_CHECK_COMPILER_HALT_ON_ERROR and *_CHECK_COMPILER_ARRAY_SIZE_NEGATIVE to *-compilers.m4 along with other *_CHECK_COMPILER_*

the socklen_t issue on this platform.

HAVE_IOCTL_SIOCGIFADDR since it's needed to use SIOCGIFADDR and Watcom C
doesn't currently define it.

upcomming work to support the broadest range of Autoconf versions

used, and define HAVE_IOCTL_SIOCGIFADDR if appropriate.