Make support for thread-unsafe C functions optional.

One of the aspects of CloudABI is that it aims to help you write code
that is thread-safe out of the box. This is very important if you want
to write libraries that are easy to reuse. For CloudABI we decided to
not provide the thread-unsafe functions. So far this is working out
pretty well, as thread-unsafety issues are detected really early on.

The following patch adds a knob to libc++,
_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS, that can be set to disable
thread-unsafe functions that can easily be avoided in practice. The
following functions are not thread-safe:

- <clocale>: locale handles should be preferred over setlocale().
- <cstdlib>: mbrlen(), mbrtowc() and wcrtomb() should be preferred over
  their non-restartable counterparts.
- <ctime>: asctime(), ctime(), gmtime() and localtime() are not
  thread-safe. The first two are also deprecated by POSIX.

Differential Revision:	http://reviews.llvm.org/D8703
Reviewed by:	marshall


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@240527 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/test/config.py

@@ -372,6 +372,8 @@ class Configuration(object):
         elif not enable_monotonic_clock:
             self.lit_config.fatal('enable_monotonic_clock cannot be false when'
                                   ' enable_threads is true.')
+        self.configure_compile_flags_no_thread_unsafe_c_functions()
+
         # Use verbose output for better errors
         self.cxx.flags += ['-v']
         sysroot = self.get_lit_conf('sysroot')
@@ -431,6 +433,15 @@ class Configuration(object):
         self.cxx.compile_flags += ['-D_LIBCPP_HAS_NO_THREADS']
         self.config.available_features.add('libcpp-has-no-threads')
 
+    def configure_compile_flags_no_thread_unsafe_c_functions(self):
+        enable_thread_unsafe_c_functions = self.get_lit_bool(
+            'enable_thread_unsafe_c_functions', True)
+        if not enable_thread_unsafe_c_functions:
+            self.cxx.compile_flags += [
+                '-D_LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS']
+            self.config.available_features.add(
+                'libcpp-has-no-thread-unsafe-c-functions')
+
     def configure_compile_flags_no_monotonic_clock(self):
         self.cxx.compile_flags += ['-D_LIBCPP_HAS_NO_MONOTONIC_CLOCK']
         self.config.available_features.add('libcpp-has-no-monotonic-clock')
@@ -522,7 +533,7 @@ class Configuration(object):
             else:
                 self.cxx.link_flags += ['-lgcc_s']
         elif target_platform.startswith('freebsd'):
-            self.cxx.link_flags += ['-lc', '-lm', '-lpthread', '-lgcc_s']
+            self.cxx.link_flags += ['-lc', '-lm', '-lpthread', '-lgcc_s', '-lcxxrt']
         else:
             self.lit_config.fatal("unrecognized system: %r" % target_platform)