package/gcc: comment on why we override the generic configure commands

The explanations given in the commit log of 7d6c79 (Compile static
versions of gcc libraries) do not explain why we have to provide custom
configure commands, instead of just adding --enable-static to the
configure options.

Add a comment in the code that explains why that is so. Add a pointer to
the ML archives with the explanations, too.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Acked-by: Jérôme Pouiller <jezz@sysmic.org>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>