Add a hook to not delete the build dir.

Either using the command line: --keep-build-dir
Or by setting an environment variable: OPAMKEEPBUILDDIR

This should fix ocaml#313

src/client/opamMain.ml

@@ -49,6 +49,7 @@ let global_args = [
   "--root"      , Arg.String set_root_dir,
     (Printf.sprintf " Change root path (default is %s)" OpamGlobals.default_opam_dir);
   "--no-checksums", Arg.Clear OpamGlobals.verify_checksums, " Do not verify checksums on download";
+  "--keep-build-dir", Arg.Set OpamGlobals.keep_build_dir, " Keep the build directory";
 ]
 
 let parse_args fn () =

src/client/opamSolution.ml

@@ -148,7 +148,7 @@ let proceed_to_install t nv =
       OpamGlobals.exit 2;
     )
   );
-  if not !OpamGlobals.debug then
+  if not (!OpamGlobals.keep_build_dir || !OpamGlobals.debug) then
     OpamFilename.rmdir build_dir
 
 let pinned_path t nv =

src/core/opamGlobals.ml

@@ -23,6 +23,11 @@ let verbose = ref (
   with _ -> false
 )
 
+let keep_build_dir = ref (
+  try Sys.getenv "OPAMKEEPBUILDDIR" <> ""
+  with _ -> false
+)
+
 let base_packages = ref true
 let verify_checksums = ref true
 let yes = ref false