Log Message:
Updated lang/coq to the newest version, 8.4pl1. Changes include:

Bug fixes

- Solved bugs :
  #2851 #2863 #2865 #2893 #2895 #2892 #2905 #2906 #2907 #2917 #2921
  #2930 #2941 #2878
- Partially fixed bug : #2904
- Various fixes concerning coq_makefile

Optimizations

- "Union by rank" optimization for universes contributed by J.H. Jourdan
  and G. Sherrer (see union-find-and-coq-universes on gagallium blog).

Libraries

- Internal organisation of some modular libraries have slightly changed
  due to bug #2904 (GenericMinMax, OrdersTac)
- No more constant "int" in ZArith/Int.v to avoid name clash with OCaml
  (cf bug #2878).

Coqide

- Improved shutdown of coqtop processes spawned by coqide
  (in particular added a missing close_on_exec primitive before forking).
- On windows, launching coqide with the -debug option now produces
  a log file in the user's temporary directory. The location of this
  log file is displayed in the "About" message.