Semi-synchronous verbose mode

As soon as there is just one sub-process running, in verbose mode, show
its output every second.

Closes #1913
This uses a SIGALRM to stop the wait at regular intervals -- disabled on Windows.
Merge pull request #1915 from OCamlPro/ssynchr-verbose

Semi-synchronous verbose mode
Fixed 4.02 warnings and safe_string
Clean build both with safe-string and older OCamls

using an OpamCompat module
Merge pull request #1916 from OCamlPro/clean-build

Clean build both with safe-string and older OCamls
resurrect compilers-to-packages script
Accept compiler definitions without upstream
More checks at resolution of the installation of base packages
Mark base packages are immovable to the solver
Add base packages to wish_install for the internal solver
Better error messages on conflicts involving base packages
Adjusting handling of base packages
Removed obsolete admin-script
Fixed the branch for cmd-engine
Fix tests for added compilers-packages precondition