Fix C++11 support in CMake and headers.

[radarsat1]

It seems that compiling software that uses C++11 against Siconos would successfully compile, but not link. It turned out be because, by default, Siconos compiles without -std=c++11, and uses boost::shared_ptr, etc. However, the use of boost::shared_ptr or std::shared_ptr is detected by the preprocessor by checking against __cplusplus in some headers. So software including those headers when -std=c++11 was enabled would incorrectly choose the std:: namespace, when Siconos was compiled with the boost:: namespace.

This patch series:

• Adds USE_CXX11 to CMake, which requests -std=c++11 enabled, and checks that it works during configuration. An error is emitted if it is requested but not supported. It is disabled by default.
• Fixes some problematic areas in mechanics where boost::array or similar was used even when c++11 was enabled.
• Replaces checks against __cplusplus in the headers with checks against SICONOS_CXXVERSION, which is hardcoded at configure time, guaranteeing that user software will select the shared_ptr type that Siconos was compiled with.

[xhub]

Good idea!

[radarsat1]

Okay I have been looking a little deeper into CMake support for C++11, and here's a slightly different proposal:

• I tried to "do the right thing" and use CMAKE_CXX_STANDARD and module WriteCompilerDetectionHeader along with target_compile_features() for detecting and specifying specific C++ features. CMake is then responsible for determining the C++ standard to use and how to use it.
• However, I discovered that mysteriously CMake doesn't provide any feature named "cxx_memory_shared_ptr" or similar -- so it's a bit of a dead-end to let CMake take care of this.

However, the idea of detecting specific features is good. So, I propose:

• User can set CMAKE_CXX_STANDARD and USE_BOOST_FOR_CXX11 to whatever he wants.
  • Therefore we presume he knows that enabling CMAKE_CXX_STANDARD=11 will enable namespace SP=std, unless USE_BOOST_FOR_CXX11 is enabled.
• If USE_BOOST_FOR_CXX11 not set, check if std::shared_ptr is supported.
  • If yes, use namespace SP=std and set a proprocessor variable SICONOS_STD_SHARED_PTR
  • Otherwise, do not use it.
• We mostly forget about SICONOS_CXXVERSION, and use feature variables in all cases.
• Variables SICONOS_STD_SHARED_PTR and SICONOS_USE_BOOST_FOR_CXX11 are recorded to SiconosConfig.h and used in headers