[boost] [Proto] Errors due to narrowing conversions in matches.hpp

Some tests of Boost.Phoenix fail on clang trunk in a C++11 mode due to narrowing conversions of non-type template arguments, which are prohibited in C++11. Specifically, the narrowing conversions occur at Line 354 and 363 in boost/proto/matches.hpp. They can be simply resolved using `static_cast<bool>(...)`. Regards, Michel