http: Don't expose function definitions when components are disabled

[captain-caveman2k]

As per other header files. For example: http.h and http_ntlm.h.

There is no point in exposing the functions definitions unnecessarily when components are disabled.

This is more of a tidy up for consistency rather than to fix an actual (existing) issue. As the current code stands these header files would only cause a problem if a libcurl developer tried to a) include them in non HTTP code or b) use the HTTP negotiate functions from outside the negotiate code in http.c - prior to this commit this would cause a linker error rather than a compilation error as the function would be defined but not exist.

[jay]

One CI error seems legit
https://travis-ci.org/curl/curl/jobs/530747966#L3427

[captain-caveman2k]

Cheers @jay

It does beg the question, why is that clean up there?

It's only the HTTP digest clean up, and none of the other HTTP authentication mechanisms (NTLM and Negotiate for example) and non off the SASL clean up functions either.

[bagder]

It fixed a bug! See 9caa3e2 (from #2272 fixed #2255)

[captain-caveman2k]

Cheers @bagder

I wonder if that bug could potentially exists for the other mechanisms?

[bagder]

Perhaps but we've had no such reports. Also, Digest is different than most of the other auth methods curl supports in that it has a state and yet survives connections so this bug can very well has been unique for Digest.