Remove build-time no-IPv6 setting

The year is 2014. All supported operating systems have IPv6 support,
most certainly at build time (this doesn't mean that IPv6 is configured,
of course).

If noone is exercising the functionality to disable it, that means it
doesn't get tested, and IMO it's better to get rid of it.

(it's also not used consistently in RPC/boost and Net code...)

configure.ac

@@ -62,13 +62,6 @@ AC_ARG_ENABLE([upnp-default],
   [use_upnp_default=$enableval],
   [use_upnp_default=no])
 
-dnl enable ipv6 support
-AC_ARG_ENABLE([ipv6],
-  [AS_HELP_STRING([--enable-ipv6],
-  [enable ipv6 (default is yes)])],
-  [use_ipv6=$enableval],
-  [use_ipv6=yes])
-
 AC_ARG_ENABLE(tests,
     AS_HELP_STRING([--enable-tests],[compile tests (default is yes)]),
     [use_tests=$enableval],
@@ -560,27 +553,6 @@ AC_MSG_RESULT($build_bitcoin_cli)
 dnl sets $bitcoin_enable_qt, $bitcoin_enable_qt_test, $bitcoin_enable_qt_dbus
 BITCOIN_QT_CONFIGURE([$use_pkgconfig], [qt4])
 
-
-if test x$use_ipv6 = xyes; then
-  dnl Check for ipv6 build requirements
-  AC_MSG_CHECKING(for operating system IPv6 support)
-  AC_TRY_LINK([
-  #if defined(_WINDOWS)
-  #include <winsock2.h>
-  #else
-  #include <sys/types.h>
-  #include <sys/socket.h>
-  #include <netinet/in.h>
-  #endif
-  ],[
-  #if !defined(_WINDOWS)
-  struct in6_addr ipv6Addr;
-  struct sockaddr_in6 addr;
-  #endif
-  int temp = socket(AF_INET6, SOCK_STREAM, 0);],
-  [AC_MSG_RESULT(yes); have_ipv6=yes; AC_DEFINE(HAVE_IPV6, 1, [Define this symbol if you have operating system IPv6 support])],
-  [AC_MSG_RESULT(no)]; have_ipv6=no)
-fi
 AC_LANG_POP
 
 if test "x$use_ccache" != "xno"; then
@@ -609,22 +581,6 @@ else
   AC_MSG_RESULT(no)
 fi
 
-dnl enable ipv6 support
-AC_MSG_CHECKING([whether to build with support for IPv6])
-if test x$have_ipv6 = xno; then
-  if test x$use_ipv6 = xyes; then
-     AC_MSG_ERROR([IPv6 requested, but cannot be built. use --disable-ipv6])
-  fi
-  AC_MSG_RESULT(no)
-else
-  if test x$use_ipv6 = xyes; then
-      AC_MSG_RESULT(yes)
-      AC_DEFINE([USE_IPV6],[1],[Define if IPv6 support should be compiled in])
-  else
-      AC_MSG_RESULT(no)
-  fi
-fi
-
 dnl enable upnp support
 AC_MSG_CHECKING([whether to build with support for UPnP])
 if test x$have_miniupnpc = xno; then
@@ -730,7 +686,6 @@ AC_SUBST(COPYRIGHT_YEAR, _COPYRIGHT_YEAR)
 
 AC_SUBST(USE_UPNP)
 AC_SUBST(USE_QRCODE)
-AC_SUBST(USE_IPV6)
 AC_SUBST(INCLUDES)
 AC_SUBST(BOOST_LIBS)
 AC_SUBST(TESTDEFS)