https://bugs.gentoo.org/855071
--- a/src/shared/CLucene/LuceneThreads.h
+++ b/src/shared/CLucene/LuceneThreads.h
@@ -7,6 +7,7 @@
 #ifndef _LuceneThreads_h
 #define  _LuceneThreads_h
 
+#include <pthread.h>
 
 CL_NS_DEF(util)
 class CLuceneThreadIdCompare;