Fix compilation after 26c4ecf31ee1 (without optimisation)
Fix compilation after 26c4ecf31ee1 (without optimisation)

We now use ceil, from math.h, which needs -lm.  This missing library
went unnoticed by most, since the compiler is smart enough to replace
`ceil' by something else from -O2 and onwards.  If we need to be able to
reliably debug, however, we still need -lm to find ceil during final
linking stage.

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