Add pkg-config support files.

These files allow compiling and linking against the reference
implementation with an invocation like `pkg-config --cflags
--libs opus`. They're optional within the opus build system,
but can simplify compilation for clients and are generally
considered part of a normal autotools build these days.

The -uninstalled version refers to local build products
within the tree, which is helpful for testing and unprivileged
builds. opus.pc is used once 'make install' has put the
libraries and headers somewhere more permanent.