Xcode can only provide autotools if it is installed, thus check that
first. Skipping this check will try to compare a `nil` Xcode version to
4.3, the first version of Xcode to not provide autotools.
FixesHomebrew/homebrew#48208.
ClosesHomebrew/homebrew#48278.
Signed-off-by: Martin Afanasjew <martin@afanasjew.de>