plan9port: modify the builder to use the INSTALL script.

plan9port ships with an INSTALL script. This commit modifies the
builder to use the script instead of a custom build script. The
commit also adds a patch to build fontsrv, which is otherwise
omitted from the build.

pkgs/tools/system/plan9port/default.nix

@@ -2,7 +2,9 @@
 
 stdenv.mkDerivation rec {
   name = "plan9port-20140228";
-  
+
+  patches = [ ./fontsrv.patch ];
+
   builder = ./builder.sh;
 
   src = fetchurl {