Don't use symbolizer on OSX

Don't use symbolizer on OSX

Summary:
When trying to build with targets files on OSX, I couldn't use the symbolizer because it needs StackTrace, which requires libunwind and elf. This makes it so that we only build on linux for now. This also makes it so that we set FOLLY_USE_SYMBOLIZER in autoconf, since that wasn't set before.

Does a few things surrounding usage of the symbolizer library:
- Introduce FOLLY_USE_SYMBOLIZER in folly-config.h and USE_SYMBOLIZER as an AM definition
-- Filter some code out of init and some other random libs that optionally need the symbolizer
- Fix libdwarf detection. Previously on a fresh ubuntu container, we didn't find libdwarf/dwarf.h, so we stopped trying before looking at dwarf.h

Reviewed By: yfeldblum

Differential Revision: D5644352

fbshipit-source-id: f0a3580c41122e5e8fdfd17a9fdbb0921be21401