Summary:
- It turns out that it's not always desirable to perform runtime
resolution of weak symbols. For example, on iOS, weak symbols are
resolved at runtime only if *all* symbol resolution is deferred util
then, which is undesirable for othe reasons. Detect such platforms at
configure time and use that information to populate detail/Malloc.h
with the correct declarations: weak symbols or extern symbols with
a value of nullptr.

Test Plan:
- fbconfig -r folly && fbmake runtests
- ./configure && make check on Ubuntu/FC/Mac

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D1002959