Don't pass -pthread to the linker on OS X.

The -pthread option is noop with GCC on Darwin, because the necessary
runtime is included in libSystem; GCC silently accepts it for
cross-platform compatibility.

Unfortunately, Clang does not and issues a warning:
clang: warning: argument unused during compilation: '-pthread'

This is arguably a bug in Clang -- see
http://llvm.org/bugs/show_bug.cgi?id=7798. But since the bug was filed
almost three years ago and nobody got around to fixing it yet, it seems
prudent to just tweak the gnu-osx toolset accordingly. Fortunately, gcc
still works just fine without -pthread, so the fix is simple enough.

src/bkl/plugins/gnu.py

@@ -200,7 +200,7 @@ def _linker_flags(self, toolset, target):
         cmd += target.type.get_link_options(target)
         if toolset.extra_link_flags:
             cmd.append(LiteralExpr(toolset.extra_link_flags))
-        if target["multithreading"]:
+        if toolset.pthread_ld_flags and target["multithreading"]:
             cmd.append(LiteralExpr(toolset.pthread_ld_flags))
         return cmd
 
@@ -436,6 +436,7 @@ class OSXGnuToolset(GnuToolset):
 
     pic_flags = None
     soname_flags = None
+    pthread_ld_flags = None
 
     def on_footer(self, file, module):
         for t in module.targets.itervalues():