netpbm: Fix a truly random build failure

Since "src" is a fetchsvn directory, the source is copied with "cp
--no-preserve=timestamps" (see commit
6d928ab684327e0eeb1bf6cd889d57ca7127e8a7).  So some source files might
get a slightly different timestamp.  Here, if lib/standard.ppmdfont
gets a newer timestamp than the generated file lib/standardppmdfont.c,
Make will try to rebuild the latter.  But that fails because the
ppmdcfont program doesn't exist (yet).

Probably stdenv should ensure that every file has the same timestamp.

pkgs/tools/graphics/netpbm/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchsvn, pkgconfig, libjpeg, libpng, flex, zlib, perl, libxml2, makeWrapper, libX11, libtiff }:
+{ stdenv, fetchsvn, pkgconfig, libjpeg, libpng, flex, zlib, perl, libxml2, makeWrapper, libX11, libtiff }:
 
 let rev = 1742; in
 stdenv.mkDerivation {
@@ -29,8 +29,11 @@ stdenv.mkDerivation {
         --subst-var-by "rgbPath1" "$out/lib/rgb.txt" \
         --subst-var-by "rgbPath2" "/var/empty/rgb.txt" \
         --subst-var-by "rgbPath3" "/var/empty/rgb.txt"
+    touch lib/standardppmdfont.c
   '';
 
+  enableParallelBuilding = true;
+
   installPhase = ''
     make package pkgdir=$PWD/netpbmpkg
     # Pass answers to the script questions