HACKING: Fix a recommendation

[Bruno Haible]

In the HACKING file, the recommendation to set CFLAGS='-O2 -m32'
contradicts what the Autoconf manual says in
https://www.gnu.org/savannah-checkouts/gnu/autoconf/manual/autoconf-2.70/html_node/Preset-Output-Variables.html
namely

  "If an option selects a 32-bit or 64-bit build on a bi-arch system, it must
   be put direcly into CC, e.g., CC='gcc -m64'. This is necessary for
   config.guess to work right."

Here's a proposed patch.

0001-HACKING-Fix-a-recommendation.patch
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