--- app/depth/Makefile.am.orig
+++ app/depth/Makefile.am
@@ -11,7 +11,8 @@
 
 libdepth_la_LIBADD = \
 	$(top_builddir)/lib/libcinepaint.la \
-	$(GTK_LIBS)
+	$(GTK_LIBS) \
+	-lm
 
 libdepth_la_SOURCES = \
 	bezier_select.c \