conditionally declare X509_V_FLAG_PARTIAL_CHAIN

Solaris has apparently backported this from 1.0.2 so our version
detection causes a compile error. This should fix it, but hopefully
we'll be able to add a Solaris builder to jenkins soon-ish.
Merge pull request #1304 from reaperhulk/fix-partial-chain-declaration

conditionally declare X509_V_FLAG_PARTIAL_CHAIN