diff --git a/src/lib/empthread/pthread.c b/src/lib/empthread/pthread.c
index a0aa4a31..fce8619e 100644
--- a/src/lib/empthread/pthread.c
+++ b/src/lib/empthread/pthread.c
@@ -33,6 +33,9 @@
  *     Markus Armbruster, 2005
  */
 
+/* Required for PTHREAD_STACK_MIN on some systems, e.g. Solaris: */
+#define _XOPEN_SOURCE 500
+
 #include <config.h>
 
 #include <stdio.h>