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/test/func/thread_alloc_external/thread_alloc_external.cc

@@ -1,3 +1,7 @@
+#ifdef SNMALLOC_USE_PTHREAD_DESTRUCTORS
+#  undef SNMALLOC_USE_PTHREAD_DESTRUCTORS
+#endif
+
 #include <snmalloc_core.h>
 #include <test/setup.h>