fix relpath computation in external_project prefix

This computation of prefix and rel_prefix was re-written in #13886
but it introduced another bug where the leading slash was missing.
In addition drive root should have trailing slash,
or it would use different path as base of relpath in some cases.

mesonbuild/modules/external_project.py

@@ -94,7 +94,7 @@ def __init__(self,
         # will install files into "c:/bar/c:/foo" which is an invalid path.
         # Work around that issue by removing the drive from prefix.
         if self.prefix.drive:
-            self.prefix = Path(relpath(self.prefix, self.prefix.drive))
+            self.prefix = Path('/' + relpath(self.prefix, self.prefix.drive + '/'))
 
         # self.prefix is an absolute path, so we cannot append it to another path.
         self.rel_prefix = Path(relpath(self.prefix, self.prefix.root))