fix 4.9 build

fix 4.9 build

Summary:
see https://www.facebook.com/groups/738749156173702/permalink/798168656898418/

I suppressed "comparison between signed and unsigned integer expressions" warnings for this first pass because there a whole lot (dozens) of those. Do we suppress those with 4.8 or something? Never seen the warnings before.

I can't find anything in boost 1.57 docs/changelog indicating why implicit conversion of boost::optional<bool> -> bool doesn't compile anymore, but it doesn't.

Test Plan: fbconfig -r --platform-all gcc-4.9-glibc-2.20 --extra-cxxflags="-Wno-sign-compare" folly && fbmake opt && fbmake runtests_opt

Reviewed By: rhysparry@fb.com

Subscribers: trunkagent, fugalh, folly-diffs@

FB internal diff: D1765093

Signature: t1:1765093:1420500411:062acf841641c7f4142c9b48e8daa45327de6822