Considering adding pkg-config support for Linux and macOS installations.

[bernardnormier]

Then users can do pkg-config --libs zeroc-ice, pkg-config --variable=slicedir zeroc-ice, and more.

To limit the number of pc files, this would require linking with -Wl,-as-needed; otherwise, all libraries get written in the resulting image. These pc files would be:

zeroc-ice
zeroc-ice++11
zeroc-freeze

ICE-7184