Making sure external C library DLLs are available at runtime is too easy to get wrong

(Imported from [Trac #866](http://hackage.haskell.org/trac/hackage/ticket/866), reported by guest on 2011-07-21)

I recently installed the hmatrix package on a Windows 7 system via using the Haskell Platform (installed only a few months ago). The hmatrix installation appeared to be successful (with an OK on foreign functions), and I can successfully compile Haskell programs that use hmatrix functions. However, when I attempt to run these programs I get the follow system error:

"The program can't start because lapack.dll is missing from your computer. Try reinstalling the program to fix this problem."

I get similar error messages if I attempt to use hmatrix functions in ghci (after importing Numeric.`LinearAlgebra?`).

The file lapack.dll is clearly visible in my folder C:\Haskell\gsl, which also includes a gsl subfolder, blas.dll, libgsl-0.dll, and libgslcblas-0.dll. I installed hmatrix with the following parameters

cabal install hmatrix --extra-lib-dir=C:\Haskell\gsl --extra-include-dir=C:\Haskell\gsl

Why is lapack.dll inaccessible at runtime despite successful installation of the package?