Add doc/ to the target in Makefile and make the cabal command customizable

[shhyou]

The commits include two changes:

1. Add doc/ prefix to the Everything.agda targets in GNUmakefile
2. Make cabal a makefile variable, thus allowing the user to substitute a different command (or designate a specific binary).

I believe (1) is a leftover of #2184. This change stops two consecutive make listings calls from re-generating the documentation.

(2) allows the user to use custom commands, e.g. using stack:

$ ln -s stack-9.10.2.yaml stack.yaml
$ stack build
$ make listings CABAL_RUN_COMMAND='stack exec' AGDA_OPTIONS=

However, fix-whitespace, check-whitespace and test seem to be cabal-specific, so I only make the cabal path customizable.

CI: shhyou#1

[JacquesCarette]

I'm fine with the output of GenerateEverything going (back) to doc/, assuming everything else keeps working. But definitely @gallais and @MatthewDaggitt need to also look at this.

[shhyou]

I believe GenerateEverything already puts Everything.agda in doc since #2184. This PR does not change its --out-dir.

[MatthewDaggitt]

This seems a reasonable set of changes to me!