Expose pthread feature flag

The code was able to use pthread destructors rather than C++ thread
local destructors.  This removes the dependence on a C++ .so on linux.
However, this is not stable on other platforms such as Apple. Where the
C++ thread local state can be cleared before the pthread destructor
runs.

src/mem/threadalloc.h

@@ -19,6 +19,7 @@
 #  if defined(SNMALLOC_THREAD_TEARDOWN_DEFINED)
 #    error At most one out of method of thread teardown can be specified.
 #  else
+#    include <pthread.h>
 #    define SNMALLOC_THREAD_TEARDOWN_DEFINED
 #  endif
 #endif
@@ -95,7 +96,7 @@ namespace snmalloc
     }
   };
 
-#  ifdef SNMALLOC_USE_PTHREAD_DESTRUCTOR
+#  ifdef SNMALLOC_USE_PTHREAD_DESTRUCTORS
   /**
    * Used to give correct signature to teardown required by pthread_key.
    */