bootstrap/5149: gcc-20011217 reads beyond EOF on cygwin

[Neil Booth]

Zack Weinberg wrote:-

> No.  But the AC_FUNC_MMAP_FILE macro (gcc/aclocal.m4) already executes
> a test program to determine whether or not mmap works the way we want.
> It would be easy enough to add a check that byte (len+1) of its test
> file is a NUL.  [And we'd better not be looking at byte len+2, or
> we'll segfault if the file length is one less than a multiple of
> PAGE_SIZE.]

Looking at the dump Werner supplied, filled with 0xc0 and NULs, I fear
this could pass on cygwin by fluke.  Better just to blacklist it IMO.
HAVE_BROKEN_MMAP or something.

Neil.