pass information about C compiler to OCaml switch

[hannesm]

please see https://discuss.ocaml.org/t/troubles-after-compiler-installed-via-opam/676/5 -- OCaml contains some magic to locate your gcc, and opam does (atm) not allow to pass the C compiler to a switch.

[dra27]

This is not entirely unrelated to Windows-stuff for opam 2 (#2930, and some stuff following it)

[bschommer]

In general wouldn't it be easier to allow the user to specify the configure flags for the ocaml compilation in some way?

[remexre]

got a similar case; can't install a musl switch (without adding extra symlinks etc) since it assumes CC=musl-gcc, rather than the correct (for my system) x86_64-linux-musl-gcc

[Jesse-Millwood]

Would this help with cross compiling too?

[hyphenrf]

what's the status on this? I've had issues installing certain opam package (eigen) with gcc due to internal compiler errors. Resorted to manually symlinking clang to coerce opam into using that.

Having the ability to say CC=clang CXX=clang++ opam ... would be much better

[kit-ty-kate]

7 years later and this issue seems to have been fixed somewhere along the line (opam 2.0.0 probably).

$ cat opam
opam-version: "2.0"
build: ["sh" "-c" "env | grep CC"]
$ CC=clang-18 opam reinstall -v .

<><> Synchronising pinned packages ><><><><><><><><><><><><><><><><><><><><><><>
[test.dev] synchronised (no changes)

The following actions will be performed:
=== recompile 1 package
  ↻ test dev (pinned)

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
⬇ retrieved test.dev  (no changes)
+ /home/kit_ty_kate/.opam/opam-init/hooks/sandbox.sh "build" "sh" "-c" "env | grep CC" (CWD=/home/kit_ty_kate/.opam/5.3/.opam-switch/build/test.dev)
- CC=clang-18
λ compiled  test.dev
⊘ removed   test.dev
∗ installed test.dev
Done.

If it still doesn't work, specifically for the compiler packages in opam-repository it should be something to change in opam-repository instead of opam (e.g. https://github.com/ocaml/opam-repository/blob/9f26a2181d2f46e23b62061679bdddb92684f204/packages/ocaml-base-compiler/ocaml-base-compiler.5.2.1/opam#L71)