The GCC 4.6.x build system breaks when run on systems with sufficiently
recent versions of texinfo. Tweak buildroot configuration to avoid this
bug.
An equivalent workaround has been commited to buildroot (commit
62322ac). We can drop this once we upgrade buildroot and/or GCC.
GCC bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60961
