configury: test the __sun macro to detect solaris OS.
recent oraclestudio compilers do not set the __sun__ macro

(cherry picked from commit open-mpi/ompi@6418d0d118aff87ac3f36b87fb951bd55de21e06)
Merge pull request #127 from ggouaillardet/topic/detect_solaris

configury: test the __sun macro to detect solaris OS.