Using MADV_FREE on Solaris/Illumos

Solaris/Illumos does not have  Linux's MADV_DONTNEED and MADV_FREE does the equivalent. This three line commit does it and works fine on SmartOS (an Illumos based distribution http://smartos.org/). 

It may be better to do in the platform specific code in configure.ac. I could not figure that out. If somebody who is more familiar with this add it there that would be great as well.