gcc: pass MAKEINFO=missing in the environment rather than as a ./configure arg

Fixes a build issue with the avr32 toolchain:

http://jenkins.free-electrons.com/job/buildroot/config=atngw100_defconfig/104/



Invalid configuration `MAKEINFO=missing': machine `MAKEINFO=missing' not
recognized

Instead pass it in the environment of ./configure, similar to how it was
done originally in 62322acb (toolchain/gcc: disable makeinfo).

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>