Index: libcxx/CMakeLists.txt
===================================================================
--- libcxx/CMakeLists.txt
+++ libcxx/CMakeLists.txt
@@ -224,6 +224,7 @@
 
 # Target options --------------------------------------------------------------
 option(LIBCXX_BUILD_32_BITS "Build 32 bit libc++." ${LLVM_BUILD_32_BITS})
+set(LIBCXX_TARGET_TRIPLE "" CACHE STRING "Use alternate target triple.")
 set(LIBCXX_SYSROOT "" CACHE STRING "Use alternate sysroot.")
 set(LIBCXX_GCC_TOOLCHAIN "" CACHE STRING "Use alternate GCC toolchain.")
 
@@ -460,11 +461,27 @@
 # 'config-ix' use them during feature checks. It also adds them to both
 # 'LIBCXX_COMPILE_FLAGS' and 'LIBCXX_LINK_FLAGS'
 add_target_flags_if(LIBCXX_BUILD_32_BITS "-m32")
-add_target_flags_if(LIBCXX_TARGET_TRIPLE "--target=${LIBCXX_TARGET_TRIPLE}")
-add_target_flags_if(LIBCXX_SYSROOT "--sysroot=${LIBCXX_SYSROOT}")
-add_target_flags_if(LIBCXX_GCC_TOOLCHAIN "--gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
-if (LIBCXX_TARGET_TRIPLE)
+
+if(LIBCXX_TARGET_TRIPLE)
+  add_target_flags("--target=${LIBCXX_TARGET_TRIPLE}")
+elseif(CMAKE_CXX_COMPILER_TARGET)
+  set(LIBCXX_TARGET_TRIPLE "${CMAKE_CXX_COMPILER_TARGET}")
+endif()
+if(LIBCXX_SYSROOT)
+  add_target_flags("--sysroot=${LIBCXX_SYSROOT}")
+elseif(CMAKE_SYSROOT)
+  set(LIBCXX_SYSROOT "${CMAKE_SYSROOT}")
+endif()
+message("TOOLCHAIN ${CMAKE_CXX_COMPILE_OPTIONS_EXTERNAL_TOOLCHAIN}")
+if(LIBCXX_GCC_TOOLCHAIN)
+  add_target_flags("--gcc-toolchain=${LIBCXX_GCC_TOOLCHAIN}")
+elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN)
+  set(LIBCXX_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
+endif()
+
+if(LIBCXX_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBCXX_TARGET_TRIPLE}")
+  set(LIBCXX_USE_TARGET ON)
 endif()
 
 # Configure compiler.
Index: libcxx/test/lit.site.cfg.in
===================================================================
--- libcxx/test/lit.site.cfg.in
+++ libcxx/test/lit.site.cfg.in
@@ -17,7 +17,7 @@
 config.configuration_variant    = "@LIBCXX_LIT_VARIANT@"
 config.host_triple              = "@LLVM_HOST_TRIPLE@"
 config.target_triple            = "@TARGET_TRIPLE@"
-config.use_target               = len("@LIBCXX_TARGET_TRIPLE@") > 0
+config.use_target               = bool("@LIBCXX_TARGET_TRIPLE@")
 config.sysroot                  = "@LIBCXX_SYSROOT@"
 config.gcc_toolchain            = "@LIBCXX_GCC_TOOLCHAIN@"
 config.generate_coverage        = "@LIBCXX_GENERATE_COVERAGE@"
Index: libcxxabi/CMakeLists.txt
===================================================================
--- libcxxabi/CMakeLists.txt
+++ libcxxabi/CMakeLists.txt
@@ -231,15 +231,26 @@
 
 # Configure target flags
 add_target_flags_if(LIBCXXABI_BUILD_32_BITS "-m32")
-add_target_flags_if(LIBCXXABI_TARGET_TRIPLE
-          "--target=${LIBCXXABI_TARGET_TRIPLE}")
-add_target_flags_if(LIBCXXABI_GCC_TOOLCHAIN
-         "--gcc-toolchain=${LIBCXXABI_GCC_TOOLCHAIN}")
-add_target_flags_if(LIBCXXABI_SYSROOT
-          "--sysroot=${LIBCXXABI_SYSROOT}")
+
+if(LIBCXXABI_TARGET_TRIPLE)
+  add_target_flags("--target=${LIBCXXABI_TARGET_TRIPLE}")
+elseif(CMAKE_CXX_COMPILER_TARGET)
+  set(LIBCXXABI_TARGET_TRIPLE "${CMAKE_CXX_COMPILER_TARGET}")
+endif()
+if(LIBCXX_GCC_TOOLCHAIN)
+  add_target_flags("--gcc-toolchain=${LIBCXXABI_GCC_TOOLCHAIN}")
+elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN)
+  set(LIBCXXABI_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
+endif()
+if(LIBCXXABI_SYSROOT)
+  add_target_flags("--sysroot=${LIBCXXABI_SYSROOT}")
+elseif(CMAKE_SYSROOT)
+  set(LIBCXXABI_SYSROOT "${CMAKE_SYSROOT}")
+endif()
 
 if (LIBCXXABI_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBCXXABI_TARGET_TRIPLE}")
+  set(LIBCXXABI_USE_TARGET ON)
 endif()
 
 # Configure compiler. Must happen after setting the target flags.
Index: libcxxabi/test/lit.site.cfg.in
===================================================================
--- libcxxabi/test/lit.site.cfg.in
+++ libcxxabi/test/lit.site.cfg.in
@@ -20,7 +20,7 @@
 config.enable_exceptions        = "@LIBCXXABI_ENABLE_EXCEPTIONS@"
 config.host_triple              = "@LLVM_HOST_TRIPLE@"
 config.target_triple            = "@TARGET_TRIPLE@"
-config.use_target               = len("@LIBCXXABI_TARGET_TRIPLE@") > 0
+config.use_target               = bool("@LIBCXXABI_TARGET_TRIPLE@")
 config.sysroot                  = "@LIBCXXABI_SYSROOT@"
 config.gcc_toolchain            = "@LIBCXXABI_GCC_TOOLCHAIN@"
 config.cxx_ext_threads          = "@LIBCXXABI_BUILD_EXTERNAL_THREAD_LIBRARY@"
Index: libunwind/CMakeLists.txt
===================================================================
--- libunwind/CMakeLists.txt
+++ libunwind/CMakeLists.txt
@@ -225,15 +225,26 @@
 endmacro()
 
 add_target_flags_if(LIBUNWIND_BUILD_32_BITS "-m32")
-add_target_flags_if(LIBUNWIND_TARGET_TRIPLE
-          "--target=${LIBUNWIND_TARGET_TRIPLE}")
-add_target_flags_if(LIBUNWIND_GCC_TOOLCHAIN
-          "--gcc-toolchain=${LIBUNWIND_GCC_TOOLCHAIN}")
-add_target_flags_if(LIBUNWIND_SYSROOT
-          "--sysroot=${LIBUNWIND_SYSROOT}")
+
+if(LIBUNWIND_TARGET_TRIPLE)
+  add_target_flags("--target=${LIBUNWIND_TARGET_TRIPLE}")
+elseif(CMAKE_CXX_COMPILER_TARGET)
+  set(LIBUNWIND_TARGET_TRIPLE "${CMAKE_CXX_COMPILER_TARGET}")
+endif()
+if(LIBUNWIND_GCC_TOOLCHAIN)
+  add_target_flags("--gcc-toolchain=${LIBUNWIND_GCC_TOOLCHAIN}")
+elseif(CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN)
+  set(LIBUNWIND_GCC_TOOLCHAIN "${CMAKE_CXX_COMPILER_EXTERNAL_TOOLCHAIN}")
+endif()
+if(LIBUNWIND_SYSROOT)
+  add_target_flags("--sysroot=${LIBUNWIND_SYSROOT}")
+elseif(CMAKE_SYSROOT)
+  set(LIBUNWIND_SYSROOT "${CMAKE_SYSROOT}")
+endif()
 
 if (LIBUNWIND_TARGET_TRIPLE)
   set(TARGET_TRIPLE "${LIBUNWIND_TARGET_TRIPLE}")
+  set(LIBUNWIND_USE_TARGET ON)
 endif()
 
 # Configure compiler.
Index: libunwind/test/lit.site.cfg.in
===================================================================
--- libunwind/test/lit.site.cfg.in
+++ libunwind/test/lit.site.cfg.in
@@ -20,7 +20,7 @@
 config.enable_exceptions        = "@LIBUNWIND_ENABLE_EXCEPTIONS@"
 config.host_triple              = "@LLVM_HOST_TRIPLE@"
 config.target_triple            = "@TARGET_TRIPLE@"
-config.use_target               = len("@LIBUNWIND_TARGET_TRIPLE@") > 0
+config.use_target               = bool("@LIBUNWIND_TARGET_TRIPLE@")
 config.sysroot                  = "@LIBUNWIND_SYSROOT@"
 config.gcc_toolchain            = "@LIBUNWIND_GCC_TOOLCHAIN@"
 config.cxx_ext_threads          = "@LIBUNWIND_BUILD_EXTERNAL_THREAD_LIBRARY@"