Make hyper opt-in, and fail when missing

[jsha]

Previously, configure would look for hyper by default, and use it if
found; otherwise it would not use hyper, and not error.

Now, configure will not look for hyper unless --with-hyper is passed. If
configure looks for hyper and fails, it will error.

Also, add -ld -lpthread -lm to Hyper's libs. I think they are required.

[bagder]

Hm, this isn't sufficient as it seems we run through this condition when the option isn't even specified so this change causes build failures. example

[bagder]

breaks the build

[jsha]

I had assumed the ./configure behavior was not to try hyper unless --with-hyper was passed, but looking again I see that the behavior is actually to try always look for hyper and use it if the library is available. What do you think of changing that, so building with hyper requires the --with-hyper flag?

Rationale: I was trying to build with Hyper yesterday, and was having trouble getting it configured correctly. I needed to make sure to build with the right feature flags and the right RUSTFLAGS, and put it in the right place, and set up LD_LIBRARY_PATH and CFLAGS=-L/usr/local/lib. That's all fine: part of the software life. But my debugging cycles were longer than necessary, because if my Hyper install was broken, ./configure --with-hyper would happily continue and set up curl without Hyper. Given that Hyper is still experimental, it seems sensible to require both the flag and the library install to turn it on. That would enable us to treat OPT_HYPER=yes as a stronger signal of user intent.

[bagder]

I had assumed the ./configure behavior was not to try hyper unless --with-hyper was passed, but looking again I see that the behavior is actually to try always look for hyper and use it if the library is available. What do you think of changing that, so building with hyper requires the --with-hyper flag?

I think it was a mistake. I think we should require the --with-hyper flag for a hyper build. In particular because the backend is still marked experimental but also because I think users expect it to behave that way!

[jsha]

Updated the change (including PR title and description) based on the conversation above.

[bagder]

Thanks!