diff --git a/acconfig.h b/acconfig.h
index 045e454d68b..3f7829f5447 100644
--- a/acconfig.h
+++ b/acconfig.h
@@ -25,6 +25,9 @@
 /* Define to `long' if <time.h> doesn't define.  */
 #undef clock_t
 
+/* Defined on Solaris to see additional function prototypes. */
+#undef __EXTENSIONS__
+
 /* Define if getpgrp() must be called as getpgrp(0). */
 #undef GETPGRP_HAVE_ARG
 
diff --git a/config.h.in b/config.h.in
index 9c0483f02fe..a390fa3faf0 100644
--- a/config.h.in
+++ b/config.h.in
@@ -90,6 +90,9 @@
 /* Define to `long' if <time.h> doesn't define.  */
 #undef clock_t
 
+/* Defined on Solaris to see additional function prototypes. */
+#undef __EXTENSIONS__
+
 /* Define if getpgrp() must be called as getpgrp(0). */
 #undef GETPGRP_HAVE_ARG
 
diff --git a/configure.in b/configure.in
index 92a5b4605bd..0818a9cd653 100644
--- a/configure.in
+++ b/configure.in
@@ -194,8 +194,13 @@ Monterey*)
     case $CC in
     cc) CC="$CC -Wl,-Bexport";;
     esac;;
+SunOS*)
+    # Some functions have a prototype only with that define, e.g. confstr
+    AC_DEFINE(__EXTENSIONS__)
+    ;;
 esac
 
+
 AC_SUBST(LIBRARY)
 AC_MSG_CHECKING(LIBRARY)
 if test -z "$LIBRARY"