rsvg-size-callback.c uses floor, so it needs to link with libm.

diff -x config.log -x config.status -ru tmp/librsvg-2.36.3/tests/Makefile.am work/librsvg-2.36.3/tests/Makefile.am
--- tmp/librsvg-2.36.3/tests/Makefile.am	2012-02-06 19:44:12.000000000 +0000
+++ work/librsvg-2.36.3/tests/Makefile.am	2012-10-07 21:23:59.291000001 +0100
@@ -34,6 +34,9 @@
 	$(top_srcdir)/rsvg-size-callback.c		\
 	$(top_srcdir)/rsvg-size-callback.h
 
+libtest_utils_la_LIBADD = \
+	$(LIBM)
+
 EXTRA_DIST =						\
 	fixtures/crash/bug620238.svg			\
 	fixtures/dimensions/bug564527.svg		\
