Fix destdir_join

The old implementation assumed a path is of Windows iff the second
character is a colon. However, that is not always true.
First, a colon can be included in a non-Windows path. For example, it is
totally fine to have a directory named ':' on Linux 5.17.0 tmpfs.
Second, a Windows path may start with \\ for UNC or extended length.

Use pathlib to handle all of these cases.

mesonbuild/scripts/__init__.py

@@ -12,10 +12,10 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from pathlib import PurePath
+
 def destdir_join(d1: str, d2: str) -> str:
     if not d1:
         return d2
     # c:\destdir + c:\prefix must produce c:\destdir\prefix
-    if len(d2) > 1 and d2[1] == ':':
-        return d1 + d2[2:]
-    return d1 + d2
+    return str(PurePath(d1, *PurePath(d2).parts[1:]))