Configure: Add support for variables in build.info files

Configure: Add support for variables in build.info files

Variables have the syntax defined with this regular expression:

    \$([[:alpha:]_][[:alnum:]_]*)

They are always local to the build.info they are defined in, and are
defined like this:

    $VAR=text

Expansion is done very simply, any reference to the variable (with the
exact same variable syntax) is replaced with its defined value.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/9144)