package/guile: link with libatomic if needed

Fix build of guile 3.0.4 with Bootlin SPARC uclibc toolchain added with
commit 1348c569d0cb7f67eca30f170b782aa8b51cc259

Fixes:
 - http://autobuild.buildroot.org/results/a72d8e14854f9c6c9632e856019a3eb8ec4818b6

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/guile/guile.mk

@@ -29,6 +29,10 @@ GUILE_CFLAGS = \
 	-DHAVE_GC_GET_FREE_SPACE_DIVISOR \
 	-DHAVE_GC_SET_FINALIZE_ON_DEMAND
 
+ifeq ($(BR2_TOOLCHAIN_HAS_LIBATOMIC),y)
+GUILE_CONF_ENV += LIBS=-latomic
+endif
+
 ifeq ($(BR2_STATIC_LIBS),y)
 GUILE_CFLAGS += -DGC_NO_DLOPEN
 endif