Only add -O to C compilations if there was -O on the command line
In 1e2b3780 I changed the option ordering for C compilations. A side effect was that -optc options came before the automatic -O we were adding, which made it so that the -debug RTS was getting optimised when it shouldn't have been. Perhaps we shouldn't have automatic -O options added to C compilations. But that might cause problems for build systems that are relying on the current behaviour, so I've made a minor change instead: now C optimisation level == Haskell optimisation level.
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