Provide path to Solaris.h include (Bug 28370)

[Joerg Sonnenberger via llvm-commits]

On Fri, Jul 01, 2016 at 12:43:59PM +0200, Rainer Orth via llvm-commits wrote:
> As described in Bug 28370, llvm/Support/Solaris.h isn't found during the
> cmake step of a VPATH build.
> 
> The following patch (already attached to the bug) fixes this by
> providing a path to the header.

LGTM

Joerg