FindBoost: use CMake's compiler version rather than querying again

Modules/FindBoost.cmake

@@ -391,13 +391,8 @@ endmacro()
 # version with a regex.
 #
 function(_Boost_COMPILER_DUMPVERSION _OUTPUT_VERSION)
-
-  exec_program(${CMAKE_CXX_COMPILER}
-    ARGS ${CMAKE_CXX_COMPILER_ARG1} -dumpversion
-    OUTPUT_VARIABLE _boost_COMPILER_VERSION
-  )
   string(REGEX REPLACE "([0-9])\\.([0-9])(\\.[0-9])?" "\\1\\2"
-    _boost_COMPILER_VERSION ${_boost_COMPILER_VERSION})
+    _boost_COMPILER_VERSION ${CMAKE_CXX_COMPILER_VERSION})
 
   set(${_OUTPUT_VERSION} ${_boost_COMPILER_VERSION} PARENT_SCOPE)
 endfunction()