Readonly shell variables containing newlines break caching

[Quinn Grier]

It appears that a configure script tries to unset all shell variables
that contain newlines in a subshell as part of the caching code. This
causes the subshell to exit early if any of the variables is readonly.
At a minimum, this breaks caching.

Here is a configure.ac file that demonstrates the issue:

      AC_INIT(p, v)
      readonly x='
      '
      AC_OUTPUT

Here is the result with Autoconf 2.69:

      $ autoconf
      $ ./configure
      ./configure: line 1699: x: readonly variable
      configure: creating ./config.status
      ./configure: line 23: x: readonly variable

If you further compare using the --cache-file option with and without
the x variable being readonly, you'll find that the cache files differ.