CGAL_Core MemoryPool<T> will always use Boost.Thread with g++

Because of bug in gcc, even if the C++11 `thread_local` keyword can be
used, it cannot be used for the TLS static member of the class template
`MemoryPool<T>`. That triggers a bug in gcc (tested with g++ 6.3.1):
```
.../include/CGAL/CORE/MemoryPool.h:113:25: error: redefinition of 'bool __tls_guard'
 MemoryPool<T, nObjects> MemoryPool<T, nObjects>::memPool;
                         ^~~~~~~~~~~~~~~~~~~~~~~
.../include/CGAL/CORE/MemoryPool.h:113:25: note: 'bool __tls_guard' previously declared here
.../include/CGAL/CORE/MemoryPool.h:113: confused by earlier errors, bailing out
Preprocessed source stored into /tmp/cc4xCWuR.out file, please attach this to your bugreport.
```

The bug seems to be from g++ >= 5:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=54948

CGAL_Core/include/CGAL/CORE/MemoryPool.h

@@ -34,10 +34,19 @@
 #ifndef _CORE_MEMORYPOOL_H_
 #define _CORE_MEMORYPOOL_H_
 
+#include <CGAL/config.h>
+#include <CGAL/tss.h>
+#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST || (defined(CGAL_HAS_THREADS)&&__GNUC__)
+// Force the use of Boost.Thread with g++ and C++11, because of the PR66944
+//   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66944
+// See also CGAL PR #1888
+//   https://github.com/CGAL/cgal/pull/1888#issuecomment-278284232
+#  include <boost/thread/tss.hpp>
+#endif
+
 #include <new>           // for placement new
 #include <cassert>
 #include <CGAL/assertions.h>
-#include <CGAL/tss.h>
 #include <vector>
 
 namespace CORE { 
@@ -81,7 +90,7 @@ public:
 
   // Access the corresponding static global allocator.
   static MemoryPool<T,nObjects>& global_allocator() {
-#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST
+#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST || (defined(CGAL_HAS_THREADS)&&__GNUC__)
     if(memPool_ptr.get() == NULL) {memPool_ptr.reset(new Self());}
     Self& memPool =  * memPool_ptr.get();
 #endif
@@ -92,7 +101,7 @@ private:
    Thunk* head; // next available block in the pool
   std::vector<void*> blocks;
 
-#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST
+#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST || (defined(CGAL_HAS_THREADS)&&__GNUC__)
   static boost::thread_specific_ptr<Self> memPool_ptr;
 #elif defined(CGAL_HAS_THREADS) // use the C++11 implementation
   static thread_local Self memPool;
@@ -101,7 +110,7 @@ private:
 #endif // not CGAL_HAS_THREADS
 };
 
-#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST
+#if CGAL_STATIC_THREAD_LOCAL_USE_BOOST || (defined(CGAL_HAS_THREADS)&&__GNUC__)
 template <class T, int nObjects >
 boost::thread_specific_ptr<MemoryPool<T, nObjects> >
 MemoryPool<T, nObjects>::memPool_ptr;

Documentation/doc/Documentation/Installation.txt

@@ -299,7 +299,7 @@ We next list the libraries and essential 3rd party software
 
 | Library   | CMake Variable | Functionality | Dependencies |
 | :-------- | :------------- | :------------ | :----------- |
-| `%CGAL`   | none           | Main library  | \sc{Gmp}, \sc{Mpfr}, \sc{Boost} (headers), Boost.Thread and Boost.System (library) for compilers not supporting the keywords `threadlocal` and the class `mutex` |
+| `%CGAL`   | none           | Main library  | \sc{Gmp}, \sc{Mpfr}, \sc{Boost} (headers), Boost.Thread and Boost.System (library) for compilers not supporting the keywords `thread_local` and the class `std::mutex` |
 | `CGAL_Core` | `WITH_CGAL_Core`  | The CORE library for algebraic numbers.\cgalFootnote{CGAL_Core is not part of \cgal, but a custom version of the \sc{Core} library distributed by \cgal for the user convenience and it has it's own license.}  | \sc{Gmp} and \sc{Mpfr} |
 | `CGAL_ImageIO` | `WITH_CGAL_ImageIO` | Utilities to read and write image files  | \sc{OpenGL}, \sc{zlib}, \sc{Vtk}(optional) |
 | `CGAL_Qt5`  | `WITH_CGAL_Qt5` | `QGraphicsView` support for \sc{Qt}5-based demos | \sc{Qt}5 and \sc{OpenGL} |
@@ -372,7 +372,11 @@ installed as binaries.
 \cgal requires the \sc{Boost} libraries. In particular the header files
 and the threading library (`Boost.Thread` and
 `Boost.System` binaries). Version 1.48 (or higher) are needed
-for compilers not supporting the keywords `threadlocal` and the class `mutex` (This is supported for \gcc 4.8 and later when using the \cpp 11 standard, and for Visual C++ starting with 2015, that is VC14).
+for compilers not supporting the keyword `thread_local` and the class `std::mutex` (This is supported for \gcc 4.8 and later when using the \cpp 11 standard, and for Visual C++ starting with 2015, that is VC14).
+
+As an exception, because of a bug in the \gcc compiler about the \cpp 11
+keyword `thread_local`, the `CGAL_Core` library always requires
+`Boost.Thread` if the \gcc compiler is used.
 
 On Windows, as auto-linking is used, you also need the binaries of
 `Boost.Serialization` and `Boost.DateTime`, but the

Installation/changes.html

@@ -147,6 +147,9 @@ and <code>src/</code> directories).
         valid commercial license for the packages they used. This can also be used
         to make sure only LGPL header files are used.
     </li>
+    <li>Because of a bug in the g++ compiler about the C++11
+    keyword <code>thread_local</code>, the CGAL_Core library now always
+    requires <code>Boost.Thread</code> if the g++ compiler is used.</li>
   </ul>
 
 <!-- New packages -->

Installation/cmake/modules/CGAL_Macros.cmake

@@ -279,7 +279,10 @@ if( NOT CGAL_MACROS_FILE_INCLUDED )
       add_to_list( CGAL_3RD_PARTY_DEFINITIONS    ${CGAL_${component}_3RD_PARTY_DEFINITIONS}    )
       add_to_list( CGAL_3RD_PARTY_LIBRARIES_DIRS ${CGAL_${component}_3RD_PARTY_LIBRARIES_DIRS} )
 
-      # Nothing to add for Core
+      if (${component} STREQUAL "Core")
+        find_package( Boost 1.48 REQUIRED thread system )
+        add_to_list( CGAL_3RD_PARTY_LIBRARIES ${Boost_LIBRARIES} )
+      endif()
 
       if (${component} STREQUAL "ImageIO")
         find_package( ZLIB QUIET )