preserve_runtime_libraries convenience PortGroup

[RJVB (René Bertin)]

This is mostly for reference though I'd love to see a similar feature in MacPorts:

Many if not most Linux distributions have one way or another to keep older versions of shared libraries installed if the newer versions introduce ABI incompatibilities. Advantages are less and typically smaller upgrades for the users, less stress on the build bots and as a result, a more reliable upgrade process. That latter claim may need explanation (and may not be totally relevant for MacPorts): building binary packages takes time, and while those builds are in process users starting an upgrade may find themselves in a situation where the new packages aren't yet available. In extreme cases that may lead to uninstallation of numerous packages for which dependency criteria are no longer met. I've experienced that personally with Kubuntu during a KDE version update; I was lucky to have taken a ZFS snapshot before starting the update.

To return closer to home: I was still running a 0.4 port:webp and thought I'd update it since it's could be a dependency for QtWebEngine (if that package allowed to build against the system webp on OS X). Webp 0.5 changes the library versioning, so a number of ports broke, including a few that are quite (very) expensive to build:

` ---> Found 11 broken files, matching files to ports ---> Found 7 broken ports:

graphviz @2.38.0 +pangocairo+universal+x11

/opt/local/bin/dot_builtins /opt/local/lib/graphviz/libgvplugin_webp.6.dylib

ImageMagick @6.9.0-0 +x11

/opt/local/lib/ImageMagick-6.9.0/modules-Q16/coders/webp.so

kde4-runtime @4.16.03.80.6 +docs

/opt/local/lib/kde4/plugins/imageformats/kimg_webp.so

opencv @3.1.0 +contrib+eigen+python27+qt5+tbb

/opt/local/lib/libopencv_imgcodecs.3.1.0.dylib

webkit-gtk @2.4.7 +video

/opt/local/lib/libwebkitgtk-1.0.0.dylib

webkit-gtk3 @2.4.7 +video

/opt/local/lib/libwebkitgtk-3.0.0.dylib

qt5-kde-devel @5.6.1 +harfbuzz+langselect+qt5kde

/opt/local/libexec/qt5/Library/Frameworks/QtWebKit.framework/Versions/5/QtWebKit /opt/local/share/qt5/plugins/imageformats/libqwebp.dylib

`

The attached PortGroup provides an easy first-approach/PoC solution to this issue: it copies the specified runtime libraries from ${prefix} to ${destroot} during the post-destroot, if they do not already exist in the destroot.

For instance, in a personal port:webp I used for testing I only needed to include the PortGroup and then add the line

` preserve_libraries [glob -nocomplain ${prefix}/lib/libwebp*.*.dylib] `

This simply allows currently installed ports to remain functional using their current dependencies and alleviates the need to revbump all dependents. They'll just move to using the new dependency version when they're rebuilt for other reasons (upgrade, reinstall, ...). I'm aware that this kind of practice may lead to runtime weirdness esp. with C++ libraries if different versions cannot coexist in a single application's runtime image. Easy enough to avoid and apart from that I see only one true issue: it won't be trivial to limit the number of old versions that will be preserved via a generic mechanism.