Update configure to look for README.md instead of README

Update configure to look for README.md instead of README

This is required after daed3a43836e. configure won't run as-is, reported
in https://codereview.chromium.org/1357773004/.

R=andybons@chromium.org

Committed: https://chromium.googlesource.com/breakpad/breakpad/+/94b1ccb51ff3ea45fc6f6226641f3d584f2190b0

[Bons, 2015-09-23 14:36:49]

lgtm

https://codereview.chromium.org/1361993002/diff/1/Makefile.in
File Makefile.in (right):

https://codereview.chromium.org/1361993002/diff/1/Makefile.in#newcode1886
Makefile.in:1886: +	autotools/compile autotools/config.guess
autotools/config.sub \
Guessing you meant to remove the README and not include README.md here.

[Mark Mentovai, 2015-09-23 14:39:59]

Yeah, I only edited configure.ac. Makefile.in and configure were auto-generated.

The README in Makefile.in was something that it includes there if it finds it.
We can add things to that list if we want, but that part of the Makefile is just
used for making a tarball of the source tree. We never do that (at least not
from the Makefile), and there are other files of ours that aren’t listed there,
so it’s not really worth doing anything.

[Mark Mentovai, 2015-09-23 14:47:06]

Committed patchset #1 (id:1) manually as
94b1ccb51ff3ea45fc6f6226641f3d584f2190b0 (presubmit successful).