build: check if -lsocket is required with *ifaddrs

build-aux/m4/l_socket.m4

@@ -0,0 +1,36 @@
+# Illumos/SmartOS requires linking with -lsocket if
+# using getifaddrs & freeifaddrs
+
+m4_define([_CHECK_SOCKET_testbody], [[
+  #include <sys/types.h>
+  #include <ifaddrs.h>
+
+  int main() {
+    struct ifaddrs *ifaddr;
+    getifaddrs(&ifaddr);
+    freeifaddrs(ifaddr);
+  }
+]])
+
+AC_DEFUN([CHECK_SOCKET], [
+
+  AC_LANG_PUSH(C++)
+
+  AC_MSG_CHECKING([whether ifaddrs funcs can be used without link library])
+
+  AC_LINK_IFELSE([AC_LANG_SOURCE([_CHECK_SOCKET_testbody])],[
+      AC_MSG_RESULT([yes])
+    ],[
+      AC_MSG_RESULT([no])
+      LIBS="$LIBS -lsocket"
+      AC_MSG_CHECKING([whether getifaddrs needs -lsocket])
+      AC_LINK_IFELSE([AC_LANG_SOURCE([_CHECK_SOCKET_testbody])],[
+          AC_MSG_RESULT([yes])
+        ],[
+          AC_MSG_RESULT([no])
+          AC_MSG_FAILURE([cannot figure out how to use getifaddrs])
+        ])
+    ])
+
+  AC_LANG_POP
+])

configure.ac

@@ -936,7 +936,7 @@ fi
 
 AC_CHECK_HEADERS([endian.h sys/endian.h byteswap.h stdio.h stdlib.h unistd.h strings.h sys/types.h sys/stat.h sys/select.h sys/prctl.h sys/sysctl.h vm/vm_param.h sys/vmmeter.h sys/resources.h])
 
-AC_CHECK_DECLS([getifaddrs, freeifaddrs],,,
+AC_CHECK_DECLS([getifaddrs, freeifaddrs],[CHECK_SOCKET],,
     [#include <sys/types.h>
     #include <ifaddrs.h>]
 )