diff --git a/test/lit.cfg b/test/lit.cfg
index dc843ac5..89d84932 100644
--- a/test/lit.cfg
+++ b/test/lit.cfg
@@ -21,10 +21,11 @@ class LibcxxTestFormat(lit.formats.FileBasedTest):
       FOO.fail.cpp - Negative test case which is expected to fail compilation.
     """
 
-    def __init__(self, cxx_under_test, cpp_flags, ld_flags):
+    def __init__(self, cxx_under_test, cpp_flags, ld_flags, exec_env):
         self.cxx_under_test = cxx_under_test
         self.cpp_flags = list(cpp_flags)
         self.ld_flags = list(ld_flags)
+        self.exec_env = dict(exec_env)
 
     def execute_command(self, command, in_dir=None):
         kwargs = {
@@ -100,7 +101,12 @@ class LibcxxTestFormat(lit.formats.FileBasedTest):
                     report += "\n\nCompilation failed unexpectedly!"
                     return lit.Test.FAIL, report
 
-                cmd = [exec_path]
+                cmd = []
+                if self.exec_env:
+                    cmd.append('env')
+                    cmd.extend('%s=%s' % (name, value)
+                               for name,value in self.exec_env.items())
+                cmd.append(exec_path)
                 if lit_config.useValgrind:
                     cmd = lit_config.valgrindArgs + cmd
                 out, err, exitCode = self.execute_command(cmd, source_dir)
@@ -154,34 +160,54 @@ if libcxx_obj_root is None:
 
 cxx_has_stdcxx0x_flag_str = lit.params.get('cxx_has_stdcxx0x_flag', None)
 if cxx_has_stdcxx0x_flag_str is not None:
-    if cxx_has_stdcxx0x_flag_str in ('1', 'True'):
+    if cxx_has_stdcxx0x_flag_str.lower() in ('1', 'true'):
         cxx_has_stdcxx0x_flag = True
-    elif cxx_has_stdcxx0x_flag_str in ('', '0', 'False'):
+    elif cxx_has_stdcxx0x_flag_str.lower() in ('', '0', 'false'):
         cxx_has_stdcxx0x_flag = False
     else:
         lit.fatal('user parameter cxx_has_stdcxx0x_flag_str should be 0 or 1')
 else:
     cxx_has_stdcxx0x_flag = getattr(config, 'cxx_has_stdcxx0x_flag', True)
 
+# This test suite supports testing against either the system library or the
+# locally built one; the former mode is useful for testing ABI compatibility
+# between the current headers and a shipping dynamic library. We require the
+# user to explicitly pick one of the two modes.
+use_system_lib_str = lit.params.get('use_system_lib', None)
+if use_system_lib_str is not None:
+    if use_system_lib_str.lower() in ('1', 'true'):
+        use_system_lib = True
+    elif use_system_lib_str.lower() in ('', '0', 'false'):
+        use_system_lib = False
+    else:
+        lit.fatal('user parameter use_system_lib should be 0 or 1')
+else:
+    use_system_lib = True
+
 # Configure extra compiler flags.
 include_paths = ['-I' + libcxx_src_root + '/include', '-I' + libcxx_src_root + '/test/support']
 library_paths = ['-L' + libcxx_obj_root + '/lib']
 compile_flags = []
 if cxx_has_stdcxx0x_flag:
-  compile_flags += ['-std=c++0x']
+    compile_flags += ['-std=c++0x']
 
 # Configure extra libraries.
+exec_env = {}
 libraries = []
 if sys.platform == 'darwin':
-  libraries += ['-lSystem']
-if sys.platform == 'linux2':
-  libraries += ['-lsupc++', '-lgcc_eh', '-lc', '-lm', '-lpthread', '-lrt', '-lgcc_s']
-  libraries += ['-Wl,-R', libcxx_obj_root + '/lib']
-  compile_flags += ['-D__STDC_FORMAT_MACROS', '-D__STDC_LIMIT_MACROS', '-D__STDC_CONSTANT_MACROS']
+    libraries += ['-lSystem']
+    if not use_system_lib:
+        exec_env['DYLD_LIBRARY_PATH'] = os.path.join(libcxx_obj_root, 'lib')
+elif sys.platform == 'linux2':
+    libraries += ['-lsupc++', '-lgcc_eh', '-lc', '-lm', '-lpthread', '-lrt', '-lgcc_s']
+    libraries += ['-Wl,-R', libcxx_obj_root + '/lib']
+else:
+    lit.fatal("unrecognized system")
 
 config.test_format = LibcxxTestFormat(
     cxx_under_test,
     cpp_flags = ['-nostdinc++'] + compile_flags + include_paths,
-    ld_flags = ['-nodefaultlibs'] + library_paths + ['-lc++'] + libraries)
+    ld_flags = ['-nodefaultlibs'] + library_paths + ['-lc++'] + libraries,
+    exec_env = exec_env)
 
 config.target_triple = None