xercesc::XMLMutex doesn't work, replace it by std::mutex
Bug: https://bugs.gentoo.org/887197

--- a/Tests/Threads/ThreadTest.cpp
+++ b/Tests/Threads/ThreadTest.cpp
@@ -49,6 +49,7 @@
 
 #if defined(XALAN_USE_THREAD_STD)
 #include <thread>
+#include <mutex>
 #elif defined(XALAN_USE_THREAD_WINDOWS)
 
 #include <process.h>
@@ -95,8 +96,8 @@
 
 
 
-typedef xercesc::XMLMutex         XMLMutexType;
-typedef xercesc::XMLMutexLock     XMLMutexLockType;
+typedef std::mutex                  XMLMutexType;
+typedef std::lock_guard<std::mutex> XMLMutexLockType;
 
 
 
@@ -121,7 +122,7 @@
 
     XMLMutexType    m_mutex;
 
-    long            m_counter;
+    volatile long   m_counter;
 };
 
 
@@ -143,7 +144,7 @@
 void
 SynchronizedCounter::increment()
 {
-    const XMLMutexLockType  theLock(&m_mutex);
+    const XMLMutexLockType  theLock(m_mutex);
 
     if (m_counter < LONG_MAX)
     {
@@ -156,7 +157,7 @@
 void
 SynchronizedCounter::decrement()
 {
-    const XMLMutexLockType  theLock(&m_mutex);
+    const XMLMutexLockType  theLock(m_mutex);
 
     if (m_counter > 0)
     {