Create extra symlinks for old SONAMES?

[mdavidsaver]

For ELF targets, currently only one qualified SONAME is installed libpvxs.so.0.1. This doesn't capture the third (maintenance) version of a build. So far I've been able to avoid removing/changing symbols, so code linked against older libpvxs can run against a newer release. But I have added some, so this scheme doesn't fail well if the opposite happens, newer code against older libpvxs.

I've wondering if a better way to handle this is to include the full version in the SONAME, and also create additional symlinks for older compatible releases?

eg. if I had done this so far, then a future 0.1.4 release would install

$ ll lib/linux-x86_64/libpvxs.so*
... lib/linux-x86_64/libpvxs.so -> libpvxs.so.0.1.4
... lib/linux-x86_64/libpvxs.so.0.1.0 -> libpvxs.so.0.1.4
... lib/linux-x86_64/libpvxs.so.0.1.1 -> libpvxs.so.0.1.4
... lib/linux-x86_64/libpvxs.so.0.1.2 -> libpvxs.so.0.1.4
... lib/linux-x86_64/libpvxs.so.0.1.3 -> libpvxs.so.0.1.4
... lib/linux-x86_64/libpvxs.so.0.1.4

(I think in Mach-O land this would be expressed as -current_version 0.1.4 -compatibility_version 0.1.0)

This PR an attempt to add some RULES to do this. eg. the preceding would be the result of:

COMPAT_VERSIONS = 0.1.0 0.1.1 0.1.2 0.1.3

@anjohnson Any thoughts on the ideas or method?