For clang toolset Boost 1.56 needs to always specify -Wno-c99-extensions

[Niall Douglas]

See mailing list for discussion, but essentially 1.56 Preprocessor turns on variadic macro support for clang which spews many megabytes of warnings on clang versions 3.2, 3.3, and 3.4. This is unacceptable for a 1.56 release, it would be embarrassing.

Adding the above warning suppression makes the problem go away.

Niall