build: Stop building gnu/build/svg.scm.

* Makefile.am (EXTRA_DIST): Add gnu/build/svg.scm.
* gnu/local.mk (GNU_SYSTEM_MODULES): Remove gnu/build/svg.scm.

It fixes the following build warnings:

gnu/build/svg.scm:36:12: warning: possibly unbound variable `cairo-create'
gnu/build/svg.scm:36:26: warning: possibly unbound variable `cairo-image-surface-create'
gnu/build/svg.scm:38:4: warning: possibly unbound variable `cairo-scale'
gnu/build/svg.scm:39:4: warning: possibly unbound variable `cairo-set-source-surface'

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

Makefile.am

@@ -4,6 +4,7 @@
 # Copyright © 2015, 2017 Alex Kost <alezost@gmail.com>
 # Copyright © 2016 Mathieu Lirzin <mthl@gnu.org>
 # Copyright © 2016, 2017 Mark H Weaver <mhw@netris.org>
+# Copyright © 2017 Mathieu Othacehe <m.othacehe@gmail.com>
 #
 # This file is part of GNU Guix.
 #
@@ -394,6 +395,7 @@ EXTRA_DIST =						\
   TODO							\
   CODE-OF-CONDUCT					\
   .dir-locals.el					\
+  gnu/build/svg.scm					\
   build-aux/build-self.scm				\
   build-aux/compile-all.scm				\
   build-aux/hydra/evaluate.scm				\

gnu/local.mk

@@ -460,7 +460,6 @@ GNU_SYSTEM_MODULES =				\
   %D%/build/linux-modules.scm			\
   %D%/build/marionette.scm			\
   %D%/build/shepherd.scm			\
-  %D%/build/svg.scm				\
   %D%/build/vm.scm				\
 						\
   %D%/tests.scm					\