Look for SuperLU headers in default locations.

Currently, the build system looks for the SuperLU headers at some hard coded locations. While those might include the default locations for system headers for many POSIX systems, other build environments might install their headers in a different default location. One example are the build environments of MSYS2 where the headers are installed, e.g. in /mingw64/include. cmake already "knows" about that default location. Adapt the find_path command to search for the SuperLU headers at the default locations first. Keep the search paths that are hard-coded currently as fall-back locations.