Update to autotools 2.72

[skangas]

This provides a more complete fix for #281 than PR #283 does. I've also included the manual steps below, which should be easily reproducible. This is an alternative to merging this patch outright (depending on your level of cautiousness after the xz backdoor).

Please see below the commit message.

Thanks!

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This essentially just does the following, with autotools 2.72 installed:

$ cp /usr/lib/share/autoconf/build-aux/config.guess \
    tool/autoconf/build-aux/config.guess
$ cp /usr/lib/share/autoconf/build-aux/config.sub \
    tool/autoconf/build-aux/config.sub
$ autoreconf
configure.ac:28: warning: The macro 'AC_LANG_C' is obsolete.
configure.ac:28: You should run autoupdate.
./lib/autoconf/c.m4:72: AC_LANG_C is expanded from...
configure.ac:28: the top level
$ autoupdate

After running autoupdate, the following manual fix was needed:

-        [m4_esyscmd_s(sed -n '/^#define MPS_RELEASE "\(.*\)"$/{s/.*"\(.*\)"/\1/;p;}' code/version.c)],
+        [m4_esyscmd_s([sed -n '/^#define MPS_RELEASE "\(.*\)"$/{s/.*"\(.*\)"/\1/;p;}' code/version.c])],