mod_proxy: follow up to r1822849 and r1822878.

Does r1822878's "static" APR_RETRIEVE_OPTIONAL_FN work if, say, mod_proxy is
builtin but mod_http2 isn't?

Not worth taking the risk here since it's not a fast path...

Note: if this is an issue, I'm afraid it applies elsewhere too.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1822879 13f79535-47bb-0310-9956-ffa450edef68