Makefile is not setting SONAME for libHalide.so

[steven-johnson]

On Linux, our Makefile produces libHalide.so without SONAME set (CMake does this for us under the hood by default). We should be consistent between the two and either set (or not) in both places.

[alexreinking]

Since we're switching to SemVer, we should definitely have SONAME set.

[alexreinking]

CMake in our latest releases sets the SONAME and creates the appropriate namelinks. I'm closing this since we aren't using the Makefile for packaging anymore and that's where this issue matters.