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.