Revert "makefiles: set CXX variable only if not overridden on command-line"

This reverts commit 72068573977f6c562a7a0928ba37a16172a57b13.

For some reason, CXX is defined by GNU make by default.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>

Created by  Steven Noonan  on September 30, 2009
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