Summary:
FOLLY_USE_SYMBOLIZER is a poor guard for the existence of execinfo.h
and backtrace(3), which is widely available on unix platforms, but not
available on Android NDK. Instead, introduce FOLLY_HAVE_BACKTRACE and
have CMake detect its availability.

Reviewed By: yfeldblum, luciang

Differential Revision: D23361495

fbshipit-source-id: 4409363f3eaa754977a93e1f006c5911d8e32020