alire_common.gpr: remove default host os to avoid mistakes on macOS (#1298)

alire_common.gpr: remove default host os to avoid mistakes on macOS (#1298)

* alire_common.gpr: remove default host os to avoid mistakes on macOS

With the default being Linux, macOS users will not notice any issue until they try to use the executable.
Even then the issue will be obscure.

* Remove any implicit detection of the host OS

This should avoid misbuilds that mix Linux/macOS.

A bash function is now provided for detection of the OS in our scripts.

---------

Co-authored-by: Alejandro R. Mosteo <amosteo@unizar.es>