[CMake] FindBoost and Mingw compiler version

[Lance Luvaul]

Hi all, why does FindBoost use the version of the currently installed 
MinGW compiler to find the Boost libs instead of the version encoded in 
the Boost lib filenames (for MinGW compiler versions 1.34 and above)?  
Doing so only necessitates the use of -DBoost_COMPILER=-mgw<ver>.  
Excerpt from FindBoost:

     elseif (MINGW)

       if(${Boost_MAJOR_VERSION}.${Boost_MINOR_VERSION} VERSION_LESS 1.34)

           set(_boost_COMPILER "-mgw") # no GCC version encoding prior to 1.34

       else()

         _Boost_COMPILER_DUMPVERSION(_boost_COMPILER_VERSION)

         set(_boost_COMPILER "-mgw${_boost_COMPILER_VERSION}")

       endif()