autoconf: Subsitute SHELL for all platforms
Establish the shell that make will use from configure. This is exactly how autoconf/automake operate, with the environment variable CONFIG_SHELL respected to override the autoconf checks. In the usual case where the user just executes `./configure', autoconf will pick a shell from the current shell, sh, bash, ksh or sh5 that meets its base criteria. The special Solaris case of looking for a POSIX shell has been changed to just set the SHELL variable since autoconf substitutes this already. The EXTRA_CONFIG_LINES substitution is dropped as it should no longer be needed.
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