Add --disable-openssl-version-check option

Add "--disable-openssl-version-check" configure option, which removes
checking for vulnerable OpenSSL versions. It is supposed to be used by
downstream packagers and distributions who have other means to ensure
vulnerabilities are fixed, such as versioned package dependencies and
vulnerability handling processes.

This avoids the necessity of editing radiusd.conf on package upgrade to
make sure it keeps working. At the same time, it provides safe default
to those installing FreeRADIUS from source.