BUG: Switch FindBoost.cmake to use CMAKE_COMPILER_IS_GNUCXX (Issue #8398)

Modules/FindBoost.cmake

@@ -532,7 +532,7 @@ ELSE (_boost_IN_CACHE)
         SET (_boost_COMPILER "-mgw${_boost_COMPILER_VERSION}")
       endif()
     elseif (UNIX)
-      if (CMAKE_COMPILER_IS_GNUCC)
+      if (CMAKE_COMPILER_IS_GNUCXX)
         if(${Boost_MAJOR_VERSION}.${Boost_MINOR_VERSION} VERSION_LESS 1.34)
           SET(_boost_COMPILER "-gcc") # no GCC version encoding prior to 1.34
         else()
@@ -563,7 +563,7 @@ ELSE (_boost_IN_CACHE)
             SET (_boost_COMPILER "-gcc${_boost_COMPILER_VERSION}")
           ENDIF()
         endif()
-      endif (CMAKE_COMPILER_IS_GNUCC)
+      endif (CMAKE_COMPILER_IS_GNUCXX)
     endif()
     if(Boost_DEBUG)
       message(STATUS "[ ${CMAKE_CURRENT_LIST_FILE}:${CMAKE_CURRENT_LIST_LINE} ] "