Index: src/gui/CMakeLists.txt
--- src/gui/CMakeLists.txt.orig
+++ src/gui/CMakeLists.txt
@@ -41,7 +41,7 @@ else()
 endif()
 
 if (HAVE_X11)
-    target_link_libraries (barrier X11)
+    target_link_libraries (barrier X11 pthread)
 endif()
 
 target_link_libraries (barrier common)
