Since Epoxy can be built with different platform-specific API, having a
way for dependent projects to discover those capabilities without
necessarily crashing the minute they attempt to use them is a good
feature to have.
We strongly direct library and application developers to use pkg-config
in order to use Epoxy, so it makes sense to add variables to the
epoxy.pc file that can be easily extracted at configuration time.
The new linker requires that the consumers of a library link against
the libraries that that library depends on, which is supported by
Libs.private.
Fixes#16