Only set AUTO{MAKE,CONF}_VERSION on OpenBSD
Only set AUTO{MAKE,CONF}_VERSION on OpenBSD

OpenBSD requires the presence of AUTOMAKE_VERSION and AUTOCONF_VERSION for
bootstrapping purposes.  Setting these on any other system requires that
explicit version to be used, rather than what might already be installed.

Therefore, only do this when the platform is OpenBSD and ignore everything
else.

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