diff --git a/src/engine/build.sh b/src/engine/build.sh
index 7cd0cdbd25..9ae0359ad7 100755
--- a/src/engine/build.sh
+++ b/src/engine/build.sh
@@ -161,6 +161,8 @@ check_toolset ()
     if test_toolset gcc && test_compiler g++$TOOLSET_SUFFIX -x c++ -std=c++11 -D_GNU_SOURCE ; then B2_TOOLSET=gcc$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     # GCC (gcc) with -pthread arg (for AIX)..
     if test_toolset gcc && test_compiler g++$TOOLSET_SUFFIX -x c++ -std=c++11 -pthread ; then B2_TOOLSET=gcc$TOOLSET_SUFFIX ; return ${TRUE} ; fi
+    # Clang (clang) on FreeBSD needs -pthread arg..
+    if test_toolset clang && test_uname FreeBSD && test_compiler clang++$TOOLSET_SUFFIX -x c++ -std=c++11 -pthread ; then B2_TOOLSET=clang$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     # Clang (clang)..
     if test_toolset clang && test_compiler clang++$TOOLSET_SUFFIX -x c++ -std=c++11 ; then B2_TOOLSET=clang$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     # Intel macOS (intel-darwin)
diff --git a/src/engine/check_cxx11.cpp b/src/engine/check_cxx11.cpp
index f413d03875..0946a45002 100644
--- a/src/engine/check_cxx11.cpp
+++ b/src/engine/check_cxx11.cpp
@@ -1,6 +1,7 @@
-/*  Copyright 2020 Rene Rivera
+/*  Copyright 2020-2023 Rene Rivera
  *  Distributed under the Boost Software License, Version 1.0.
- *  (See accompanying file LICENSE.txt or https://www.bfgroup.xyz/b2/LICENSE.txt)
+ *  (See accompanying file LICENSE.txt or
+ * https://www.bfgroup.xyz/b2/LICENSE.txt)
  */
 
 /*
@@ -11,23 +12,36 @@ compiler to build the engine with.
 */
 
 // Some headers we test...
-#include <thread>
 #include <memory>
+#include <mutex>
+#include <thread>
 
 // Some basic constexpr support?
-template <int C> struct X { static constexpr int c = C; };
+template <int C>
+struct X
+{
+	static constexpr int c = C;
+};
 
 int main()
 {
-    // Check for basic thread calls.
-    // [2020-08-19] Mingw-w64 with win32 threading model (as opposed to posix
-    // threading model) does not really have std::thread etc. Please see comments
-    // in sysinfo.cpp.
-    #ifndef _WIN32
-    { auto _ = std::thread::hardware_concurrency(); }
-    #endif
+#ifndef _WIN32
+	// Check for basic thread calls.
+	// [2020-08-19] Mingw-w64 with win32 threading model (as opposed to posix
+	// threading model) does not really have std::thread etc. Please see
+	// comments in sysinfo.cpp.
+	{
+		auto _ = std::thread::hardware_concurrency();
+	}
+	{
+		std::mutex m;
+		std::lock_guard<std::mutex> l(m);
+	}
+#endif
 
-    // [2021-08-07] We check the following C++11 features: brace initialization,
-    // unique_ptr. Plus the author's ability to memorize some digits.
-    { const std::unique_ptr <float> pf {new float {3.14159f}}; }
+	// [2021-08-07] We check the following C++11 features: brace initialization,
+	// unique_ptr. Plus the author's ability to memorize some digits.
+	{
+		const std::unique_ptr<float> pf { new float { 3.14159f } };
+	}
 }