netpbm: disable parallel build on Tiger

parallel build on Tiger is fragile and fails
on multi-processor machines, perhaps due to the
use of gmake? disable parallel build fixes it

graphics/netpbm/Portfile

@@ -286,6 +286,7 @@ subport libnetpbm {
 platform darwin 8 {
     depends_build-append port:gmake
     build.cmd ${prefix}/bin/gmake
+    use_parallel_build  no
     pre-configure {
         set fl [open "| ${configure.cc} --version"]
         set data [read $fl]