various improvements over incremental build

[bobzhang]

a bunch of improvment including:

• no unneeded rebuild for cmi when changing package specs

• better postbuild support, (built in $out_last support)

previously for each target, the postbuild command has to be repeated, we introduced $out_last to remove such duplication
old

rule mij
  command = $bsc $g_pkg_flg -bs-package-output commonjs:lib/js/$in_d:.js $g_lib_incls $warnings $bsc_flags -o $out $in $postbuild
  dyndep = $in_e.d
  restat = 1
o src/demo.cmj | src/demo.cmi ../js/src/demo.js : mij src/demo.mlast || src/demo.d
   postbuild = && cat ../js/src/demo.js

rule mij
  command = $bsc $g_pkg_flg -bs-package-output commonjs:lib/js/$in_d:.js $g_lib_incls $warnings $bsc_flags -o $out $in && cat $out_last
  dyndep = $in_e.d
  restat = 1
o src/demo.cmj | src/demo.cmi ../js/src/demo.js : mij src/demo.mlast || src/demo.d

• some micro-tweaks

TODO: rule variables are expanded when used, if we don't use the dynamism we should interoplate directly