configure script does not accept non-absolute GHC variable

Summary

Passing ./configure GHC=ghc-9.8.2 silently switches back to whatever ghc is in PATH, potentially leading to further compile failures.

This is a result of the combination of the following autotools macros:

AC_ARG_VAR(GHC,[Use as the full path to GHC. [default=autodetect]])
AC_PATH_PROG([GHC], [ghc])

In my opinion, we should be using AC_CHECK_PROG([GHC], [ghc], [ghc]). I see no reason for the "absolute path" requirement, especially given the silent and possibly wrong fallback.

Steps to reproduce

1. unpack GHC
2. run ./configure GHC=ghc-9.8.2 while some other ghc is the PATH default
3. observe the configure script picking the ghc in PATH as opposed to what was explicitly specified

Expected behavior

Either fail properly if GHC is a non-absolute path variable or relax the constraints. I propose the latter.

Environment

• GHC version used: any recent

Optional:

• Operating System: Linux
• System Architecture: x86_64