Skip compiling diagrams which were not discovered in the first pass.

This could happen if a diagram URL got removed by CPP, in which case
we will see it in the (dumb) second pass, but should not compile it
since it will not actually appear in the final Haddock output.  Fixes
#7.
Merge pull request #13 from diagrams/issue7

Skip compiling diagrams which were not discovered in the first pass.