Be able to use +32bit variant of compiler

[dinosaure]

Currently. gcc does not have the -m32 option to be able to compile a switch with +32bit. It could be useful to have this specific gcc to test libraries with a 32-bit runtime. Thanks!

[dinosaure]

This is the error when I want to have 4.07.1+32bit:

#=== ERROR while compiling ocaml-variants.4.07.1+32bit ========================#

# context     2.0.3 | linux/x86_64 |  | git://github.com/ocaml/opam-repository

# path        ~/.opam/4.07.1+32bit/.opam-switch/build/ocaml-variants.4.07.1+32bit

# command     ~/.opam/opam-init/hooks/sandbox.sh build ./configure -prefix /home/travis/.opam/4.07.1+32bit -with-debug-runtime -cc gcc -m32 -as as --32 -aspp gcc -m32 -c -host i386-linux -partialld ld -r -melf_i386

# exit-code   2

# env-file    /tmp/opam-travis-5527/ocaml-variants-5527-9088e2.env

# output-file /tmp/opam-travis-5527/ocaml-variants-5527-9088e2.out

### output ###

# Configuring OCaml version 4.07.1

# Configuring for host i386-pc-linux-gnu ...

# Configuring for target i386-pc-linux-gnu ...

# Using compiler gcc -m32.

# Compiler family and version: gcc-5-4.

# [ERROR!] Unable to compile the test program.

#  Make sure the C compiler gcc -m32 is properly installed.

<><> Error report <><><><><><><><><><><><><><><><><><><><><><><><><><><><><><><>

┌─ The following actions failed

│ λ build ocaml-variants 4.07.1+32bit

└─ 

┌─ The following changes have been performed (the rest was aborted)

│ ∗ install base-bigarray base

│ ∗ install base-threads  base

│ ∗ install base-unix     base

└─ 

# Run eval $(opam env) to update the current shell environment

Switch initialisation failed: clean up? ('n' will leave the switch partially

installed) [Y/n] y

The command "bash -ex .travis-opam.sh" exited with 31.

Done. Your build exited with 1.