Unbreak build against Boost 1.67

[jbeich]

Regressed by boostorg/thread@1e84b978b2bb. Found downstream (error log).

$ cat foo.cc
#include <boost/thread.hpp>

void foo() {}

int main()
{
  boost::thread thr(foo);
  return 0;
}

$ clang++ -fuse-ld=lld foo.cc -I/usr/local/include -L/usr/local/lib/ -lboost_thread -lboost_system
/usr/bin/ld.lld: error: undefined symbol: pthread_condattr_init
>>> referenced by foo.cc
>>>               /tmp/foo-060c75.o:(boost::pthread::cond_init(pthread_cond*&))

/usr/bin/ld.lld: error: undefined symbol: pthread_condattr_setclock
>>> referenced by foo.cc
>>>               /tmp/foo-060c75.o:(boost::pthread::cond_init(pthread_cond*&))

/usr/bin/ld.lld: error: undefined symbol: pthread_condattr_destroy
>>> referenced by foo.cc
>>>               /tmp/foo-060c75.o:(boost::pthread::cond_init(pthread_cond*&))

[wwmayer]

Merged.