Suppress clang warnings when compiling bundled boost 1.56

Clang generates a bunch of warnings of the type:
bundled/boost-1.56.0/include/boost/preprocessor/tuple/eat.hpp:23:26: warning: variadic macros are a C99 feature [-Wvariadic-macros]
bundled/boost-1.56.0/include/boost/mpl/if.hpp:131:1: warning: empty macro arguments are a C99 feature [-Wc99-extensions]

This patch just disables those warnings. Also see https://code.google.com/p/dealii/issues/detail?id=252