Reference a store path of an input?

[Jim Newsome <jim@sporksmith.net>]

I'm just getting started experimenting with guix and guile, so I'm 
likely missing something obvious.

I'm trying to write a package definition for some software that builds 
with cmake and uses glib. The software searches some paths explicitly to 
find the glib headers etc; it should be fixed to use pkg-config instead, 
but I'd like to figure out how to work around it both as a learning 
exercise and so that I can work around it before I can get a fix merged.

I'd like to add some explicit flags to the cmake invocation so that it 
knows where to find glib. What do I substitute in for "get-store-path" 
below? Pointers to how I might have found the appropriate helper or 
approach on my own also appreciated.

```
(define-public oniontrace
   (package
     (name "oniontrace")
     (build-system cmake-build-system)
     (inputs `(("glib", glib)))
     (arguments
       '(#:configure-flags
          (list (string-append "-DCMAKE_EXTRA_INCLUDES=" (get-store-path 
  "glib") "/include"))))
...
```