Broken build with cmake ≧ 3.1

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This took me a while to track down:

• 3.0.2: nvi2 builds normally

• 3.1.3: error,

  -- Build files have been written to: /Users/marc/builds/nvi2.alt/build
  Scanning dependencies of target headers
  make[2]: *** No rule to make target `/Users/marc/builds/nvi2.alt/cl/extern.h', needed by `CMakeFiles/headers'.  Stop.
  make[1]: *** [CMakeFiles/headers.dir/all] Error 2
  make: *** [all] Error 2
  

• 3.2.1: same error

Issue: In 3.1.3 and 3.2.1, extern.h headers aren't generated before running the main build rules (headers custom target).

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(It's not clear whether this is a regression in cmake.)