diff --git a/libpkgconf/config.h.meson b/libpkgconf/config.h.meson
index 406f283..d3498f5 100644
--- a/libpkgconf/config.h.meson
+++ b/libpkgconf/config.h.meson
@@ -66,3 +66,8 @@
 #mesondefine SYSTEM_INCLUDEDIR
 #mesondefine SYSTEM_LIBDIR
 #mesondefine PERSONALITY_PATH
+
+/* Enable Solaris extensions. */
+#ifndef __EXTENSIONS__
+# define __EXTENSIONS__ 1
+#endif