configure test for a mmap flag

[Steve Kargl]

gfortran uses mmap for its IO if an OS has mmap.  If
found, mmap is used with the MAP_SHARED flag.  On linux
(and other OS's) this seems to be okay, but on FreeBSD
IO can significantly degrade if a file pre-exists.  In
some testing I've done, linux's MAP_SHARED appears to be
equivalent to FreeBSD's (MAP_SHARED | MAP_NOSYNC).  I
need to add a configure test to determine if MAP_NOSYNC
is present.  I haven't found an example in the gcc/
directory hierarachy on how this might be accomplished.
Any pointers?

-- 
Steve