[Agda] agda ghc backend executable (re)location

[Martin Stone Davis]

I would like to `agda --compile` but have agda use an 
alternatively-installed ghc. On my machine, agda is using `/usr/bin/ghc` 
but I would like it to use a different executable. I have tried (1) 
creating a bash alias and (2) creating a bash script named `ghc`. In 
both tries, when I run `ghc` from the command line, it invokes my 
preferred executable, but agda still uses the one in `/usr/bin`. Is 
there a way to convince agda to use an alternate?

By way of background, I am running Arch Linux and I have had ghc-related 
problems when trying to install the `master` version of agda. I resolved 
this by using `stack` to install agda. The current issue is that, when I 
tried to `agda --compile hello-world.agda`, I received errors with the 
ieee package that I could not easily resolve. My plan is to use stack to 
install ghc, and thus I need agda to refer to this alternative ghc 
executable.