Replace the `--with-compiler` configure script argument with HC=/path/to/hc

[23Skidoo]

See #2946, especially #2946 (comment). Basically, the ./configure --with-compiler interface that we use to pass the GHC location to the configure script is non-standard and error-prone, and should be phased out in favour of HC=.

[hvr]

Re compatibility, does a custom-setup specification apply to build-type: configure as well?

If so, we wouldn't need to keep --with-compiler around (and could get rid of that warning sooner), as we could fixup those 3 packages on Hackage via meta-data updates.

[23Skidoo]

Looks like the custom-setup section is ignored when the build-type is not Custom.