build broken on FreeBSD

[Alizter]

This is a regression found on the 3.21 release candidate branch: #12920 :

chrome-trace.3.21.0~alpha3 is not installed. Install it? [Y/n] y
The following actions will be performed:
=== install 2 packages
  ∗ chrome-trace 3.21.0~alpha3 (pinned)
  ∗ dune         3.21.0~alpha3          [required by chrome-trace]

<><> Processing actions <><><><><><><><><><><><><><><><><><><><><><><><><><><><>
⬇ retrieved chrome-trace.3.21.0~alpha3, dune.3.21.0~alpha3  (cached)
[ERROR] The compilation of dune.3.21.0~alpha3 failed at "ocaml boot/bootstrap.ml -j 39".

#=== ERROR while compiling dune.3.21.0~alpha3 =================================#
# context              2.5.0 | freebsd/x86_64 | ocaml-base-compiler.5.4.0 | file:///home/opam/opam-repository
# path                 ~/.opam/5.4.0/.opam-switch/build/dune.3.21.0~alpha3
# command              ~/.opam/5.4.0/bin/ocaml boot/bootstrap.ml -j 39
# exit-code            2
# env-file             ~/.opam/log/dune-2183-1fb933.env
# output-file          ~/.opam/log/dune-2183-1fb933.out
### output ###
# ocamlc -output-complete-exe -intf-suffix .dummy -g -o .duneboot.exe -I boot -I +unix unix.cma boot/types.ml boot/libs.ml boot/duneboot.ml
# ./.duneboot.exe -j 39
# cd _boot && /home/opam/.opam/5.4.0/bin/ocamlopt.opt -c -g -I +unix -I +threads lmdb_stubs.c -ccopt -I .
# vendor/ocaml-lmdb/lmdb_stubs.c:21:10: fatal error: 'alloca.h' file not found
#    21 | #include <alloca.h>
#       |          ^~~~~~~~~~
# 1 error generated.

https://opam.ci.ocaml.org/github/ocaml/opam-repository/commit/a95df9014bc79103fde668cf2adbe6680fd2f9cf/variant/freebsd,freebsd-14.3-ocaml-5.4-amd64,chrome-trace.3.21.0~alpha3

The stubs we have for the lmdb cache don't compile on FreeBSD without tweaking the includes. A fix can be found here:

• fix FreeBSD build #12926

This is a (non-critical) regression in 3.21.

[shonfeder]

Why do you think this is “non-critical”?

[shonfeder]

We chatted on slack. Thanks, @Alizter.

I think this should be a blocking fix for the release, as introducing a regression that breaks availability on a tier-2 supported platform without need seems inappropriate to stable software.