Don't run OS initialization during elaboration

As this makes impossible to tweak such initialization with `alr config`.

src/alire/alire-platforms-current.ads

@@ -31,6 +31,15 @@ package Alire.Platforms.Current is
 
    function Operating_System return Platforms.Operating_Systems;
 
+   ----------------------
+   --  Self configuration
+
+   procedure Initialize;
+   --  Do any initialization that is necessary for this platform. This is
+   --  called as soon as we know the user is not running `alr config`, as we
+   --  want to allow the opportunity to configure things without triggering
+   --  this initialization.
+
    --------------------------------
    -- Portable derived utilities --
    --------------------------------

src/alire/os_freebsd/alire-platforms-current__freebsd.adb

@@ -31,4 +31,10 @@ package body Alire.Platforms.Current is
    function Operating_System return Alire.Platforms.Operating_Systems
    is (Alire.Platforms.FreeBSD);
 
+   ----------------
+   -- Initialize --
+   ----------------
+
+   procedure Initialize is null;
+
 end Alire.Platforms.Current;

src/alire/os_linux/alire-platforms-current__linux.adb

@@ -133,4 +133,10 @@ package body Alire.Platforms.Current is
    function Operating_System return Alire.Platforms.Operating_Systems
    is (Alire.Platforms.Linux);
 
+   ----------------
+   -- Initialize --
+   ----------------
+
+   procedure Initialize is null;
+
 end Alire.Platforms.Current;

src/alire/os_macos/alire-platforms-current__macos.adb

@@ -74,4 +74,10 @@ package body Alire.Platforms.Current is
 
    function Operating_System return Platforms.Operating_Systems is (MacOS);
 
+   ----------------
+   -- Initialize --
+   ----------------
+
+   procedure Initialize is null;
+
 end Alire.Platforms.Current;

src/alire/os_windows/alire-platforms-current__windows.adb

@@ -421,6 +421,13 @@ package body Alire.Platforms.Current is
 
    end Setup_Msys2;
 
-begin
-   Setup_Msys2;
+   ----------------
+   -- Initialize --
+   ----------------
+
+   procedure Initialize is
+   begin
+      Setup_Msys2;
+   end Initialize;
+
 end Alire.Platforms.Current;

src/alr/alr-commands-config.ads

@@ -4,9 +4,11 @@ package Alr.Commands.Config is
 
    type Command is new Commands.Command with private;
 
+   Command_Name : constant String := "config";
+
    overriding
    function Name (Cmd : Command) return CLIC.Subcommand.Identifier
-   is ("config");
+   is (Command_Name);
 
    overriding
    procedure Execute (Cmd  : in out Command;

src/alr/alr-commands.adb

@@ -516,7 +516,19 @@ package body Alr.Commands is
 
       Create_Alire_Folders;
 
+      declare
+         use AAA.Strings;
       begin
+         --  Once we know the user is not trying to configure, run the
+         --  platform-specific initialization (which may rely on such config).
+         if To_Lower_Case (Sub_Cmd.What_Command) /= Config.Command_Name
+         then
+            Alire.Platforms.Current.Initialize;
+            Trace.Debug ("Platform-specific initialization done.");
+         else
+            Trace.Debug
+              ("Platform-specific initialization skipped (alr config).");
+         end if;
 
          Set_Builtin_Aliases;