fix undefined variable in windows build

[cserteGT3]

Building the package on Windows failed with:

ERROR: LoadError: UndefVarError: `mpath` not defined
Stacktrace:
 [1] find_lib_ker()
   @ Main C:\Users\username\.julia\dev\MathLink\deps\build.jl:77
 [2] top-level scope
   @ C:\Users\username\.julia\dev\MathLink\deps\build.jl:112
 [3] include(fname::String)
   @ Base.MainInclude .\client.jl:489
 [4] top-level scope
   @ none:5
in expression starting at C:\Users\username\.julia\dev\MathLink\deps\build.jl:99

I fixed the variable names, now it builds and works fine.