./build.sh needlessly recompiles OPAM dependencies

[jeehoonkang]

./build.sh's line 9 tries to recompile OPAM dependencies even if they are already installed. I hope we can just skip it.

$ opam install sparrow --yes --deps-only


=-=- Synchronising pinned packages =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
[sparrow] /home/jeehoonkang/Works/sparrow/ already up-to-date
The following actions will be performed:
  ∗  install   conf-perl     1             
  ∗  install   ppx_tools     5.0           
  ∗  install   camlidl       1.05          
  ∗  install   result        1.2           
  ∗  install   bson          0.89.3        
  ∗  install   base-num      base          
  ∗  install   conf-gmp      1             
  ∗  install   ppx_core      113.33.01+4.03
  ∗  install   ppx_deriving  4.1           
  ∗  install   lymp          0.2.3         
  ∗  install   num           0             
  ∗  install   conf-mpfr     1             
  ∗  install   ppx_optcomp   113.33.01+4.03
  ↻  recompile coq           8.6           
  ↻  recompile cil           1.7.3         
  ↻  recompile batteries     2.5.3         
  ∗  install   mlgmpidl      1.2.4         
  ∗  install   ppx_driver    113.33.02+4.03
  ∗  install   apron         20160125      
  ∗  install   ppx_type_conv 113.33.02+4.03
  ∗  install   ppx_compare   113.33.00+4.03
===== ∗  18   ↻  3 =====
...