makefile: use atomic target for contribs

This is not the ideal solution, but it avoids an infinite loop, so…

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

Created by  Steven Noonan  on October 25, 2009
CDH37HWAVXOPATZC7GW4R6VJZRNKLDLCPUFJF364GOT25OBXLO2AC
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