libnetpbm.so uses libm functions so we need to link with libm

patch by Diego Pettenò
http://bugs.gentoo.org/121253

--- netpbm-10.31/lib/Makefile
+++ netpbm-10.31/lib/Makefile
@@ -115,7 +115,7 @@
 	$(SYMLINK) $< $@
 libnetpbm.$(NETPBMLIBSUFFIX).$(MAJ).$(MIN): $(LIBOBJECTS) $(LIBOBJECTS_X)
 	$(LD) $(LDSHLIB) -o $@ $(LIBOBJECTS) $(LIBOBJECTS_X) \
-          $(SHLIB_CLIB) $(LADD)
+          $(SHLIB_CLIB) $(LADD) -lm
 endif
 
 ifeq ($(NETPBMLIBTYPE),dll)