FOLLY_SANITIZE_ADDRESS cause inconsistencies

[Roman Gershman]

Hi,

If folly  is built in release mode (without sanitizer enabled) - it, obviously, does not produce sanitizer related code.

However when my application is built with sanitizer enabled and linked with folly it fails with linker error 

"folly/include/folly/fibers/FiberManagerInternal-inl.h:70: undefined reference to `folly::fibers::FiberManager::registerStartSwitchStackWithAsan(void**, void const*, unsigned long)'"

because the decision whether to call sanitizer-related code is inside a header file that is compiled during when my application is built.

[Roman Gershman]

I think the decision on whether folly should sanitize should be propagated via configure arguments and be consistent once built (config.h?) and not deduced via compiled flags.