#if FOLLY_HAVE_INT128_T rather than #ifdef
#if FOLLY_HAVE_INT128_T rather than #ifdef

Summary: Switch from #ifdef to #if so the configuration can define the macro to 0

Reviewed By: yfeldblum, Orvid

Differential Revision: D3838748

fbshipit-source-id: e287b07f0fdfdc86c882538e96f2078795b85bfd

charsyam/folly,rklabs/folly,floxard/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,Orvid/folly,floxard/folly,rklabs/folly,facebook/folly,floxard/folly,charsyam/folly,Orvid/folly,charsyam/folly,facebook/folly,Orvid/folly,facebook/folly,rklabs/folly,charsyam/folly,Orvid/folly,Orvid/folly,floxard/folly,floxard/folly