Build glitch in hslibs/contrib

[Jeff Lewis]

The Makefile in hslibs/contrib tries to expand the heap for Cubic_Spline
by setting:
    Cubic_Spline_HC_OPTS=-H10m
This relies on
    HC_OPTS = ... $($*_HC_OPTS) ...
However, since the source file is in the subdirectory src, $* gets
expanded to `src/Cubic_Spline', instead of the intended `Cubic_Spline',
the end result being that ghc bails out with the dreaded
    GHC's heap exhausted!

--Jeff