Fix build error after make clobber

Forgot to make the CHPL_CONFIG_CHECK target depend
upon the bin/platform directory being created. Whoops!

[trivial and not reviewed]



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@21853 3a8e244f-b0f2-452b-bcba-4c88e055c3ca