Cabal: tell configure scripts which compiler is in use

[andreabedini]

Fixes #7452. First step of #2947.

runConfigureScript passes --with-compiler= to a package's configure script, but takes the
value from configHcPath, and cabal-install never sets that: it hands the compiler over as a
program path override (--ghc=/path) and leaves configHcPath empty on purpose. So the script
gets the flavour name and, following autoconf convention, goes looking for ghc on PATH.

With cabal-install 3.16.1.0 and a configure script that just echoes its arguments:

$ cabal build -w ~/.ghcup/bin/ghc-9.12.2
configure args: --with-compiler=ghc --prefix=... CC=/usr/lib64/ccache/gcc CXX=/usr/lib64/ccache/g++
GHC=<unset>

A script that needs the actual compiler runs the wrong one. GHC's own ghc-internal does
exactly that ($GHC --print-prim-module to generate GHC/Internal/Prim.hs) and fails under any
other ghc, which is how I ran into it: building GHC's stage2 libraries with the stage1 compiler
via cabal picked up the ghc on PATH.

The fix takes the compiler and its package tool from the configured ProgramDb when the flags
do not name them, and passes them as:

• --with-compiler=/path and --with-hc-pkg=/path (what existing scripts look at);
• HC=/path as a configure argument, the standard autoconf form suggested in Replace the --with-compiler configure script argument with HC=/path/to/hc #2947, alongside
  the existing CC=/CXX=;
• GHC and GHC_PKG in the environment, for GHC, for scripts that use AC_ARG_VAR([GHC])
  rather than an option.

Explicit --with-compiler/--with-hc-pkg flags are left as given. With the fix:

configure args: --with-compiler=/home/andrea/.ghcup/bin/ghc-9.12.2 --with-hc-pkg=/home/andrea/.ghcup/ghc/9.12.2/bin/ghc-pkg-9.12.2 ... CC=... CXX=... HC=/home/andrea/.ghcup/bin/ghc-9.12.2
GHC=/home/andrea/.ghcup/bin/ghc-9.12.2

I have not removed --with-compiler (the second half of #2947); the six or so packages on
Hackage that implement it still work, and dropping it is a separate compatibility decision.

Testing

New cabal-testsuite/PackageTests/ConfigureCompiler: a build-type: Configure package whose
hand-written configure checks that --with-compiler/--with-hc-pkg are executable paths,
that HC= matches --with-compiler, that GHC/GHC_PKG are set, and that the compiler runs.
It fails on master ([ -x ghc ] is false) and passes with the change. The existing Configure
tests (Configure, BuildTypeConfigure, ConfigureCXX, ConfigureComponent) still pass.
Skipped on Windows (POSIX shell script).

QA notes

Create a package with build-type: Configure whose configure script prints its arguments and
$GHC, then cabal build -w /full/path/to/ghc. Before: --with-compiler=ghc, GHC unset.
After: the full path in --with-compiler, --with-hc-pkg, HC= and GHC.

[Copilot]

🟡 Changes recommended

The new HC= and GHC/GHC_PKG exports can disagree with an explicitly provided --with-compiler/--with-hc-pkg, causing inconsistent inputs to configure scripts.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR fixes how build-type: Configure packages learn which Haskell compiler Cabal is using, ensuring configure scripts receive the actual configured compiler path (not just the flavor name like ghc) and adding a regression test to prevent future breakage.

Changes:

• Recover compiler and package-tool paths from the configured ProgramDb and pass them to configure via --with-compiler, --with-hc-pkg, HC=..., and (for GHC) GHC/GHC_PKG env vars.
• Add a new testsuite package (PackageTests/ConfigureCompiler) whose configure script asserts the above are executable paths and that the compiler runs.
• Document the behavior change in the changelog.

File summaries

File	Description
changelog.d/configure-script-compiler.md	Changelog entry describing the new configure script compiler-path propagation.
Cabal/src/Distribution/Simple/ConfigureScript.hs	Implements compiler/path recovery from ProgramDb and exports it to configure via args and env.
cabal-testsuite/PackageTests/ConfigureCompiler/configure-compiler.cabal	New minimal build-type: Configure test package.
cabal-testsuite/PackageTests/ConfigureCompiler/configure	New configure script asserting correct --with-compiler/HC=/env behavior.
cabal-testsuite/PackageTests/ConfigureCompiler/cabal.test.hs	New testsuite driver (skips on Windows, runs v2-build).
cabal-testsuite/PackageTests/ConfigureCompiler/cabal.project	New test project file for the testsuite package.
cabal-testsuite/PackageTests/ConfigureCompiler/cabal.out	Golden output for the new test.
cabal-testsuite/PackageTests/ConfigureCompiler/A.hs	Minimal library module for the test package.

Review details

Suppressed comments (1)

Cabal/src/Distribution/Simple/ConfigureScript.hs:222

• HC= is currently derived from mHcPath (the ProgramDb value) rather than the effective --with-compiler value passed via configureArgs cfg'. This can make HC= disagree with --with-compiler when configHcPath is explicitly set. Prefer using configHcPath cfg' so all configure-script spellings stay in sync.

          -- The standard autoconf spelling for the Haskell compiler (#2947);
          -- '--with-compiler' above is kept for the scripts that use it.
          ++ ["HC=" ++ hcPath | Just hcPath <- [mHcPath]]

• Files reviewed: 8/8 changed files
• Comments generated: 1
• Review effort level: Lite

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