diff -up ClanLib-2.3.7/configure.ac~ ClanLib-2.3.7/configure.ac
--- ClanLib-2.3.7/configure.ac~	2021-08-02 14:15:48.000000000 +0200
+++ ClanLib-2.3.7/configure.ac	2021-08-02 14:23:20.421967995 +0200
@@ -259,6 +259,7 @@ fi
 
 if test "$WIN32" = "no"; then
 	extra_CFLAGS_clanCore="$extra_CFLAGS_clanCore -pthread"
+	extra_LIBS_clanCore="$extra_LIBS_clanCore -lpthread"
 else
 	extra_CFLAGS_clanCore="$extra_CFLAGS_clanCore -mthreads -std=c++0x"
 fi
diff -up ClanLib-2.3.7/Sources/Core/System/mutex.cpp~ ClanLib-2.3.7/Sources/Core/System/mutex.cpp
--- ClanLib-2.3.7/Sources/Core/System/mutex.cpp~	2011-04-06 17:33:13.000000000 +0200
+++ ClanLib-2.3.7/Sources/Core/System/mutex.cpp	2021-08-02 15:52:07.631909837 +0200
@@ -30,19 +30,6 @@
 #include "API/Core/System/mutex.h"
 #include "API/Core/System/exception.h"
 
-#ifndef WIN32
-// We need to do this because the posix threads library under linux obviously
-// suck:
-extern "C"
-{
-#if defined(__APPLE__) || defined (__FreeBSD__) || defined(__OpenBSD__)
-	int pthread_mutexattr_settype(pthread_mutexattr_t *attr, int kind);
-#else
-	int pthread_mutexattr_setkind_np(pthread_mutexattr_t *attr, int kind);
-#endif
-}
-#endif
-
 /////////////////////////////////////////////////////////////////////////////
 // CL_Mutex Construction:
 
@@ -53,15 +40,7 @@ CL_Mutex::CL_Mutex()
 #else
 	pthread_mutexattr_t attr;
 	pthread_mutexattr_init(&attr);
-	#if defined(__FreeBSD__) || defined(__APPLE__)
-	pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
-	#else
-	#if PTHREAD_MUTEX_RECURSIVE_NP
-	pthread_mutexattr_setkind_np(&attr, PTHREAD_MUTEX_RECURSIVE);
-	#else
-	pthread_mutexattr_setkind_np(&attr, PTHREAD_MUTEX_RECURSIVE_NP);
-	#endif
-	#endif
+	pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE_NP);
 	pthread_mutex_init(&handle, &attr);
 	pthread_mutexattr_destroy(&attr);
 #endif