Fix old GCC ABI check in CMake package config (pytorch#72081)

Summary:
Pull Request resolved: pytorch#72081

This PR fixes the libstdc++ ABI check in CMake package configuration file (i.e. `TorchConfig.cmake`) The `_GLIBCXX_USE_CXX11_ABI` flag is a property of `libstdc++`, not GNU compiler collection. In its current form C++ libraries built with Clang on Linux fail since the `torch` CMake target propagates `_GLIBCXX_USE_CXX11_ABI` only when used with gcc.
ghstack-source-id: 148056323

Test Plan: Built a dummy C++ library that depends on libtorch with both gcc and clang on Linux

Reviewed By: malfet

Differential Revision: D33899849

fbshipit-source-id: 3e933b2c7a17d1fba086caa8aaec831223760882
(cherry picked from commit 41d18c6)

caffe2/CMakeLists.txt

@@ -1142,7 +1142,7 @@ endif()
   endif()
 
   # XXX This ABI check cannot be run with arm-linux-androideabi-g++
-  if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+  if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
     if(DEFINED GLIBCXX_USE_CXX11_ABI)
       message(STATUS "_GLIBCXX_USE_CXX11_ABI is already defined as a cmake variable")
     else()

cmake/TorchConfig.cmake.in

@@ -166,8 +166,8 @@ if(@USE_CUDA@)
   list(APPEND TORCH_LIBRARIES ${TORCH_CUDA_LIBRARIES})
 endif()
 
-# When we build libtorch with the old GCC ABI, dependent libraries must too.
-if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
+# When we build libtorch with the old libstdc++ ABI, dependent libraries must too.
+if(CMAKE_SYSTEM_NAME STREQUAL "Linux")
   set(TORCH_CXX_FLAGS "-D_GLIBCXX_USE_CXX11_ABI=@GLIBCXX_USE_CXX11_ABI@")
 endif()