ENV.gcc: search HOMEBREW_PREFIX/bin for gcc-4.2
ENV.gcc: search HOMEBREW_PREFIX/bin for gcc-4.2

Allow the use of a Homebrewed gcc-4.2 in order to support homebrew-alt's
apple-gcc42 formula.

This also removes the :force filtering for the gcc-4.2 check error.

Closes Homebrew/homebrew#9384.

Signed-off-by: Misty De Meo <85d6b3741948526aebc363d97f74b6575b2cc027@gmail.com>

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