scons/MSYS2-MinGW-W64: Fix build options defaults

Signed-off-by: Prodea Alexandru-Liviu <liviuprodea@yahoo.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Cc: <mesa-stable@lists.freedesktop.org>

When building in a MSYS2 Mingw-w64 environment Mesa3D sets wrong default build options which inevitably lead to build failure.

(cherry picked from commit 6309c31f)