diff --git a/libraries/liblmdb/mdb.c b/libraries/liblmdb/mdb.c
index 79e96e2bbd..cecd1da6df 100644
--- a/libraries/liblmdb/mdb.c
+++ b/libraries/liblmdb/mdb.c
@@ -142,6 +142,10 @@ typedef SSIZE_T	ssize_t;
 /* Most platforms have posix_memalign, older may only have memalign */
 #define HAVE_MEMALIGN	1
 #include <malloc.h>
+/* On Solaris, we need the POSIX sigwait function */
+#if defined (__sun)
+# define _POSIX_PTHREAD_SEMANTICS	1
+#endif
 #endif
 
 #if !(defined(BYTE_ORDER) || defined(__BYTE_ORDER))