Made config check for boost.thread add -pthread to compile and link flags.

This should fix asio check failing with undefined references to libpthread
on some systems.

scons/boost.py

@@ -67,6 +67,8 @@ def CheckBoost(context, boost_lib, require_version = None, header_only = False):
         env.AppendUnique(CPPPATH = [boostdir], LIBPATH = [boostlibdir])
     if not header_only:
         env.AppendUnique(LIBS = [libname])
+    if boost_lib == "thread" and env["PLATFORM"] == "posix":
+        env.AppendUnique(CCFLAGS = ["-pthread"], LINKFLAGS = ["-pthread"])
 
     test_program = """
         #include <boost/%s>