Index: CMakeLists.txt
===================================================================
--- CMakeLists.txt
+++ CMakeLists.txt
@@ -57,6 +57,8 @@
    This option may only be used when LIBCXX_ENABLE_THREADS=OFF." ON)
 option(LIBCXX_INSTALL_HEADERS "Install the libc++ headers." ON)
 option(LIBCXX_INSTALL_SUPPORT_HEADERS "Install libc++ support headers." ON)
+set(LIBCXX_SYSROOT "" CACHE STRING "Use alternate sysroot.")
+set(LIBCXX_GCC_TOOLCHAIN "" CACHE STRING "Use alternate GCC toolchain.")
 if (LIBCXX_BUILT_STANDALONE)
   set(LLVM_USE_SANITIZER "" CACHE STRING
       "Define the sanitizer used to build the library and tests")
@@ -277,6 +279,11 @@
   endif()
 endif()
 
+list(APPEND LIBCXX_CXX_FLAGS "-target ${LIBCXX_TARGET_TRIPLE}")
+append_if(LIBCXX_CXX_FLAGS LIBCXX_SYSROOT "--sysroot ${LIBCXX_SYSROOT}")
+append_if(LIBCXX_CXX_FLAGS LIBCXX_GCC_TOOLCHAIN
+          "-gcc-toolchain ${LIBCXX_GCC_TOOLCHAIN}")
+
 string(REPLACE ";" " " LIBCXX_CXX_FLAGS "${LIBCXX_CXX_FLAGS}")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${LIBCXX_CXX_FLAGS}")
 
Index: test/libcxx/test/config.py
===================================================================
--- test/libcxx/test/config.py
+++ test/libcxx/test/config.py
@@ -229,9 +229,9 @@
         locale.setlocale(locale.LC_ALL, default_locale)
 
         # Write an "available feature" that combines the triple when
-        # use_system_cxx_lib is enabled. This is so that we can easily write XFAIL
-        # markers for tests that are known to fail with versions of libc++ as
-        # were shipped with a particular triple.
+        # use_system_cxx_lib is enabled. This is so that we can easily write
+        # XFAIL markers for tests that are known to fail with versions of
+        # libc++ as were shipped with a particular triple.
         if self.use_system_cxx_lib:
             self.config.available_features.add(
                 'with_system_cxx_lib=%s' % self.config.target_triple)
@@ -287,7 +287,7 @@
         if not enable_threads:
             self.configure_compile_flags_no_threads()
             if not enable_monotonic_clock:
-                self.configure_compile_flags_no_monotonic_clock() 
+                self.configure_compile_flags_no_monotonic_clock()
         elif not enable_monotonic_clock:
             self.lit_config.fatal('enable_monotonic_clock cannot be false when'
                                   ' enable_threads is true.')
@@ -297,6 +297,15 @@
         compile_flags_str = self.get_lit_conf('compile_flags', '')
         self.compile_flags += shlex.split(compile_flags_str)
 
+        sysroot = self.get_lit_conf('sysroot')
+        if sysroot:
+            self.compile_flags += ['--sysroot', sysroot]
+        gcc_toolchain = self.get_lit_conf('gcc_toolchain')
+        if gcc_toolchain:
+            self.compile_flags += ['-gcc-toolchain', gcc_toolchain]
+
+        self.compile_flags += ['-target', self.config.target_triple]
+
     def configure_compile_flags_header_includes(self):
         self.compile_flags += ['-I' + self.libcxx_src_root + '/test/support']
         libcxx_headers = self.get_lit_conf('libcxx_headers',
Index: test/lit.site.cfg.in
===================================================================
--- test/lit.site.cfg.in
+++ test/lit.site.cfg.in
@@ -14,6 +14,9 @@
 config.use_sanitizer            = "@LLVM_USE_SANITIZER@"
 config.abi_library_path         = "@LIBCXX_CXX_ABI_LIBRARY_PATH@"
 config.configuration_variant    = "@LIBCXX_LIT_VARIANT@"
+config.target_triple            = "@LIBCXX_TARGET_TRIPLE@"
+config.sysroot                  = "@LIBCXX_SYSROOT@"
+config.gcc_toolchain            = "@LIBCXX_GCC_TOOLCHAIN@"
 
 # Let the main config do the real work.
 lit_config.load_config(config, "@LIBCXX_SOURCE_DIR@/test/lit.cfg")