Build-dependency on guile-json

[Justus Winter]

Hi,

README says:
>   - optionally 
> [[http://savannah.nongnu.org/projects/guile-json/][Guile-JSON]], for the 
> 'guix import pypi' command

However, lacking guile-json, building guix fails with:

  LOAD     (guix import github)
Backtrace:
In ice-9/boot-9.scm:
1724: 19 [%start-stack load-stack ...]
1729: 18 [#<procedure 1a789c0 ()>]
In unknown file:
   ?: 17 [primitive-load 
"/home/teythoon/repos/guix/./build-aux/compile-all.scm"]
In ice-9/eval.scm:
 432: 16 [eval # #]
In ice-9/boot-9.scm:
 775: 15 [for-each #<procedure load-module-file (a)> #]
2864: 14 [resolve-interface (guix import github) #:select ...]
2789: 13 [#<procedure 19c1780 at ice-9/boot-9.scm:2777:4 (name #:optional 
autoload version #:key ensure)> # ...]
3065: 12 [try-module-autoload (guix import github) #f]
2401: 11 [save-module-excursion #<procedure 3f3e930 at ice-9/boot-9.scm:3066:17 
()>]
3085: 10 [#<procedure 3f3e930 at ice-9/boot-9.scm:3066:17 ()>]
In unknown file:
   ?: 9 [primitive-load-path "guix/import/github" ...]
In ice-9/eval.scm:
 505: 8 [#<procedure 190e500 at ice-9/eval.scm:499:4 (exp)> (define-module # # 
...)]
In ice-9/psyntax.scm:
1106: 7 [expand-top-sequence ((define-module # # # ...)) () ((top)) ...]
 989: 6 [scan ((define-module (guix import github) #:use-module ...)) () ...]
 279: 5 [scan ((#(syntax-object let # ...) (#) (# #) ...)) () ...]
In ice-9/eval.scm:
 411: 4 [eval # ()]
In ice-9/boot-9.scm:
2951: 3 [define-module* (guix import github) #:filename ...]
2926: 2 [resolve-imports (((ice-9 match)) ((srfi srfi-1)) ((json)) ...)]
2867: 1 [resolve-interface (json) #:select ...]
In unknown file:
   ?: 0 [scm-error misc-error #f "~A ~S" ("no code for module" (json)) #f]

ERROR: In procedure scm-error:
ERROR: no code for module (json)
Makefile:4716: recipe for target 'make-go' failed
make[2]: *** [make-go] Error 1


Cheers,
Justus