Allow the syntax of the .include directive to optionally have '='

If the old openssl versions not supporting the .include directive
load a config file with it, they will bail out with error.

This change allows using the .include = <filename> syntax which
is interpreted as variable assignment by the old openssl
config file parser.

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8141)

(cherry picked from commit 9d556033)