When looking at lib/Basic/Targets/OSTargets.h, I noticed that _REENTRANT is defined
unconditionally on Solaris, unlike all other targets and what either Studio cc (only define
it with -mt) or gcc (only define it with -pthread) do.
This patch follows that lead.
Differential Revision: https://reviews.llvm.org/D41241
llvm-svn: 332343