diff --git a/config.cmake.h.in b/config.cmake.h.in
index d2c2f204..b028f4c9 100644
--- a/config.cmake.h.in
+++ b/config.cmake.h.in
@@ -176,8 +176,10 @@
 
 #ifndef _XOPEN_SOURCE
 #cmakedefine DODEFINE_XOPEN_SOURCE 500
+#ifdef DODEFINE_XOPEN_SOURCE
 #define _XOPEN_SOURCE DODEFINE_XOPEN_SOURCE
 #endif
+#endif
 
 /* Enable threading extensions on Solaris.  */
 #ifndef _POSIX_PTHREAD_SEMANTICS
@@ -190,8 +192,10 @@
 /* Enable general extensions on Solaris.  */
 #ifndef __EXTENSIONS__
 #cmakedefine DODEFINE_EXTENSIONS
+#ifdef DODEFINE_EXTENSIONS
 #define __EXTENSIONS__ DODEFINE_EXTENSIONS
 #endif
+#endif
 
 
 /* Target processor is big endian. */