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Add recognition of compiler version and prefix for clang. Accommodate changes to version numbering scheme for GCC 5+ and clang 4+ - Minor number becomes patch, so excluded it from compiler prefix. - Improves searching for Boost 1.69+ libraries built with GCC 5+ and clang 4+, where library names are generated with compiler tag based on major version only eg. -gcc5 for GC 5.5.0 or -clang6 for clang 6.0.0 - Follows up related changes in upcoming Boost 1.69 https://github.com/boostorg/build/pull/349 Dump detected compiler version in Boost_DEBUG message. Replace use of CMAKE_COMPILER_IS_GNUCXX with CMAKE_CXX_COMPILER_ID.
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