autoconf 2.52: configure scripts should not remove "core"

[Jonathan Kamens]

Configure scripts generated by autoconf 2.52 recursively remove "core"
before exiting.

Since the script can't know for a fact that it's responsible for
creating "core", it shouldn't remove it.  Furthermore, it can't know
for a fact that the package it's compiling doesn't have a real file or
directory named "core".

I just ran into this in the build tree of a package which I work on --
it has a subdirectory named "Core", which is no problem on Linux, but
when we attempt to configure on Windows (under Cygwin), that whole
subdirectory gets removed (because "core" == "Core" on Windows) when
the configure script exits!  Ack!

At the very least, it should do "rm -f core" instead of "rm -rf core"
so that "core" will only be removed if it is a directory rather than a
file.

(If you respond to this bug report, please make sure to CC me
directly, because I don't read the bug-autoconf list.)

Thanks,

  Jonathan Kamens