Lack of flexibility in pointing the path to the libtorch system library

[benjamasu]

At the moment the LIBTORCH_LIB and LIBTORCH_INCLUDE variables imply that there are appropriate directories under them. But on systems where torch is built and installed in a non-standard way, there are difficulties in specifying appropriate paths without creating symbolic links.

For example, Gentoo, where on 64-bit systems libtorch is installed under /usr/lib64 and headers under /usr/include/torch.

It would be nice to be able to specify direct paths, without the requirement of finding libraries and headers under lib and include respectively. At least be able to manually specify the appropriate CFLAGS.