Support Clang 3.4.2

[eschnett]

The cmake code that autodetects Clang's version number expects only two digits (e.g. "3.4"). Clang 3.4.2 uses three digits. I suggest the enclosed patch.

  --- cmake/FindHPX_Boost.cmake
  +++ cmake/FindHPX_Boost.cmake
  @@ -27,7 +27,7 @@
           ARGS ${CMAKE_CXX_COMPILER_ARG1} --version
           OUTPUT_VARIABLE BOOST_COMPILER_VERSION_NUMBER)

  -      string(REGEX REPLACE "clang version ([0-9])\\.([0-9]) .*" "\\1\\2"
  +      string(REGEX REPLACE "clang version ([0-9])\\.([0-9])(\\.[0-9])? .*" "\\1\\2"
           BOOST_COMPILER_VERSION_NUMBER ${BOOST_COMPILER_VERSION_NUMBER})
       elseif(NOT MSVC)
         exec_program(${CMAKE_CXX_COMPILER}
  EOT