proposed fix for PR bootstrap/15093 [for trunk]

[Jim Wilson]

Joe Buck wrote:
> 	* configure.ac: Test for existence of mmap before checking
>           blacklist.  Fix suggested by Jim Wilson.

I checked this in also.  I got a surprising amount of configure changes, 
even though I used the same autoconf-2.59 version that it had been 
previously generated with.  I don't know why.

Again, I don't believe any libffi copyright assignment is needed here 
because

For the record, the same change was also suggested by Aaron Ucko, and he 
beat me by almost 3 hours.
-- 
Jim Wilson, GNU Tools Support, http://www.SpecifixInc.com