Force BOOST_PP_VARIADIC for clang

Current clang has full support for all variadic macro feature required
by Boost.Preprocessor. However, older Boost turned off variadic feature
for clang by default, probably because clang was broken at that time. It
should be safe to turn it on now.

CMakeLists.txt

@@ -98,6 +98,13 @@ if(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLANGXX)
     endif()
 endif(CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLANGXX)
 
+if(CMAKE_COMPILER_IS_CLANGXX)
+    # older boost.preprocessor turn off variadics for clang
+    add_definitions(-DBOOST_PP_VARIADICS=1)
+    message(STATUS "Force BOOST_PP_VARIADICS=1 for clang")
+endif()
+
+
 # ================================================================================
 # Output directories for install target