autoconf: Subsitute SHELL for all platforms

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.