Implement pkg-config --variables equivalent

[dcbaker]

CPS cps-org/cps#4 is a relevant question here.

There are a few cases that I know of that need to be solved:

• To get the location of a binary tool with Qt you run pkg-config QtCore --variable bindir, then append the binary you want
• To get the location of a binary with wayland_scanner you run pkg-config wayland-scanner --variable wayland_scanner
• I see a number of pkgs looking at things like libdir, bindir, libexedir, as well as some custom directories like pluginsdir
• One X11 package uses this to distinguish whether it was built with thread-local support

Some of this is custom data, but some of this may well just want to act as an interface proxy in cps-config to point to existing data, the wayland_scanner example might just somehow reference the wayland_scanner component.