How to force OpensSL to use SHA2 to generate random numbers?

By default, OpenSSL uses the md_rand generator, which uses the MD5 hash as the pseudorandom function.

Knowing that OpenSSL PRNG options are : (see http://ift.tt/1v3fG4g page 12)

• Hash
• HMAC
• CTR AES

How do I change that to one of the SHA2 family functions? or maybe HMAC?