[runtimes] Deprecate FOO_SYSROOT & friends

As suggested in https://reviews.llvm.org/D112155.

Differential Revision: https://reviews.llvm.org/D119836

GitOrigin-RevId: 641a141da1f22de319e891bdfd61e614632b39f0
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f40ee88..46c8c26 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -132,6 +132,16 @@
 set(LIBCXXABI_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}" CACHE PATH "GCC toolchain for cross compiling.")
 set(LIBCXXABI_SYSROOT "${CMAKE_SYSROOT}" CACHE PATH "Sysroot for cross compiling.")
 
+if(NOT CMAKE_SYSROOT AND LIBCXXABI_SYSROOT)
+  message(WARNING "LIBCXXABI_SYSROOT is deprecated, please use CMAKE_SYSROOT instead")
+endif()
+if(NOT CMAKE_CXX_COMPILER_TARGET AND LIBCXXABI_TARGET_TRIPLE)
+  message(WARNING "LIBCXXABI_TARGET_TRIPLE is deprecated, please use CMAKE_CXX_COMPILER_TARGET instead")
+endif()
+if(NOT CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN AND LIBCXXABI_GCC_TOOLCHAIN)
+  message(WARNING "LIBCXXABI_GCC_TOOLCHAIN is deprecated, please use CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN instead")
+endif()
+
 set(LIBCXXABI_LIBCXX_LIBRARY_PATH "" CACHE PATH "The path to libc++ library.")
 set(LIBCXXABI_LIBRARY_VERSION "1.0" CACHE STRING
 "Version of libc++abi. This will be reflected in the name of the shared \