diff --git a/configure.ac b/configure.ac
index 4113bc78..b37129a5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -46,15 +46,19 @@ AC_CONFIG_AUX_DIR([build-aux])
 AC_CONFIG_MACRO_DIR([m4])
 AC_CONFIG_HEADERS([config.h])
 
-
 ### Checks for programs
 AC_PROG_AWK
 AC_PROG_INSTALL
 AC_PROG_RANLIB
-AC_PROG_CC
+
+AC_PROG_CC_STDC
 AM_PROG_CC_C_O
 # Private automake macro, tsk, tsk, tsk...
 _AM_DEPENDENCIES(CC)
+AC_DEFINE([_XOPEN_SOURCE], [600],
+	  [Request POSIX-1.2001 with XSI Extension])
+# Note: this is after AC_PROG_CC_STDC, because Solaris cc fails with
+# _XOPEN_SOURCE unless switched to C99, which breaks AC_PROG_CC_STDC.
 
 AC_ARG_VAR(NROFF, [nroff command])
 AC_CHECK_PROG(NROFF, groff, [GROFF_NO_SGR= groff -Tascii -U], nroff)
@@ -168,6 +172,7 @@ main(int argc, char *argv[])
 ### Checks for library functions
 
 AC_CHECK_FUNCS(getaddrinfo)
+MY_WORKING_IN6_IS_ADDR_V4MAPPED
 MY_FUNC_MAKECONTEXT
 
 
diff --git a/m4/my_working_in6_is_addr_v4mapped.m4 b/m4/my_working_in6_is_addr_v4mapped.m4
new file mode 100644
index 00000000..2c476826
--- /dev/null
+++ b/m4/my_working_in6_is_addr_v4mapped.m4
@@ -0,0 +1,20 @@
+AC_DEFUN([MY_WORKING_IN6_IS_ADDR_V4MAPPED], [
+    AC_CACHE_CHECK([whether IN6_IS_ADDR_V4MAPPED works],
+		   my_cv_func_in6_is_addr_v4mapped, [
+	AC_COMPILE_IFELSE(
+	    [AC_LANG_SOURCE([[#include <netinet/in.h>
+int foo(struct sockaddr_in6 *sap6)
+{
+    return IN6_IS_ADDR_V4MAPPED(&sap6->sin6_addr);
+}]])],
+	    [my_cv_func_in6_is_addr_v4mapped=yes],
+	    [my_cv_func_in6_is_addr_v4mapped=no])])
+    AC_CHECK_MEMBERS([struct sockaddr_in6.sin6_addr.s6_addr],
+		     [my_member_sockaddr_in6_sin6_addr_s6_addr=yes],
+		     [my_member_sockaddr_in6_sin6_addr_s6_addr=no],
+		     [[#include <netinet/in.h>]])
+    if test "x$my_cv_func_in6_is_addr_v4mapped$my_member_sockaddr_in6_sin6_addr_s6_addr" = xyesyes; then
+	AC_DEFINE([HAVE_WORKING_IN6_IS_ADDR_V4MAPPED], [1],
+		  [Define if you have a working IN6_IS_ADDR_V4MAPPED()])
+    fi
+])
diff --git a/src/client/configure.ac b/src/client/configure.ac
index 00aac39e..76268544 100644
--- a/src/client/configure.ac
+++ b/src/client/configure.ac
@@ -44,6 +44,9 @@ AC_INIT([Wolfpack Empire Client],
 AC_CONFIG_SRCDIR([empire.6])
 AC_CONFIG_HEADERS([config.h])
 
+AC_DEFINE([_XOPEN_SOURCE], [600],
+	  [Request POSIX-1.2001 with XSI Extension])
+
 
 ### Checks for programs.
 AC_PROG_CC
diff --git a/src/lib/common/rdsched.c b/src/lib/common/rdsched.c
index a98ec0a4..05b518c2 100644
--- a/src/lib/common/rdsched.c
+++ b/src/lib/common/rdsched.c
@@ -30,9 +30,6 @@
  *     Markus Armbruster, 2007-2011
  */
 
-/* Required for strptime() */
-#define _XOPEN_SOURCE 500
-
 #include <config.h>
 
 #include <ctype.h>
diff --git a/src/lib/empthread/pthread.c b/src/lib/empthread/pthread.c
index 2d8b5092..3e02132d 100644
--- a/src/lib/empthread/pthread.c
+++ b/src/lib/empthread/pthread.c
@@ -33,9 +33,6 @@
  *     Ron Koenderink, 2007-2009
  */
 
-/* Required for PTHREAD_STACK_MIN on some systems, e.g. Solaris: */
-#define _XOPEN_SOURCE 500
-
 #include <config.h>
 
 #include <errno.h>
diff --git a/src/lib/global/constants.c b/src/lib/global/constants.c
index 252a8a96..1e09e586 100644
--- a/src/lib/global/constants.c
+++ b/src/lib/global/constants.c
@@ -29,7 +29,7 @@
  *  Known contributors to this file:
  *     Ken Stevens, 1995
  *     Steve McClure, 1996
- *     Markus Armbruster, 2004-2014
+ *     Markus Armbruster, 2004-2020
  */
 
 #include <config.h>
@@ -41,7 +41,11 @@ char *privname = "Deity forgot to edit econfig";
 /* E-mail of the deity */
 char *privlog = "careless@invalid";
 /* Divine hosts and networks */
-char *privip = "127.0.0.1 ::1";
+char *privip = "127.0.0.1 ::1"
+#ifndef HAVE_WORKING_IN6_IS_ADDR_V4MAPPED
+    " ::ffff:127.0.0.1"		/* See sockaddr_ntop() */
+#endif
+    ;
 
 char *post_crash_dump_hook = "";
 
diff --git a/src/lib/player/accept.c b/src/lib/player/accept.c
index a631f79b..0ec2a93f 100644
--- a/src/lib/player/accept.c
+++ b/src/lib/player/accept.c
@@ -28,7 +28,7 @@
  *
  *  Known contributors to this file:
  *     Dave Pare, 1994
- *     Markus Armbruster, 2005-2014
+ *     Markus Armbruster, 2005-2020
  */
 
 #include <config.h>
@@ -228,10 +228,12 @@ sockaddr_ntop(struct sockaddr *sap, char *buf, size_t bufsz)
     else {
 	sap6 = (struct sockaddr_in6 *)sap;
 	addr = &sap6->sin6_addr;
+#ifdef HAVE_WORKING_IN6_IS_ADDR_V4MAPPED
 	if (IN6_IS_ADDR_V4MAPPED(&sap6->sin6_addr)) {
 	    af = AF_INET;
 	    addr = sap6->sin6_addr.s6_addr + 12;
 	}
+#endif
     }
     return inet_ntop(af, addr, buf, bufsz);
 #else