adjusted Makefile to allow for provided required LAPACK source files rather than downloading them

This simple patch allows you to provide the LAPACK source files (`lapack-3.4.2.tgz`, `timing.tgz` and `large.tgz`), rather than making the `Makefile` download them.

This is useful when the system you are testing on doesn't have access to the web, e.g., because of some security policy.