makefile: force regeneration of compflag.h and build.h on revision change

The build.h and compflag.h files stayed around regardless of whether or not they were dependencies of the .INTERMEDIATE pseudo-target. This led to stale headers.

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

Created by  Steven Noonan  on October 19, 2009
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