Rename hwaci-error to the more descriptive hwaci-fatal. Use autosetup's file-isexec...

Rename hwaci-error to the more descriptive hwaci-fatal. Use autosetup's file-isexec instead of [file executable] for portability. Remove the binary file lookup cache - unnecessary complexity. When searching for tools like tclsh, check under $prefix/bin before checking the $PATH. This seems like the right thing to do, but the fact that autosetup's file-search API's do not do that by default leaves some room for doubt about the wisdom of this change.

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