Typo in build helpers: `MAXOSX_DEPLOYEMENT_TARGET`

build-helpers/i686-apple-darwin11-g++ and build-helpers/i686-apple-darwin11-gcc have a typo:

-DMAXOSX_DEPLOYEMENT_TARGET=10.6

There are actually two bugs: MAXOSX not MACOSX and DEPLOYEMENT not DEPLOYMENT.

Since this define has probably never had an effect, I suppose it can be removed.

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