external_project: workaround for MinGW

On Python 3.12, self.prefix.relative_to(self.prefix.drive) no longer works and raises error like:
ValueError: 'C:/msys64/mingw64' is not in the subpath of 'C:'

mesonbuild/modules/external_project.py

@@ -93,7 +93,7 @@ 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 = self.prefix.relative_to(self.prefix.drive)
+            self.prefix = Path('/' + str(self.prefix.relative_to(self.prefix.drive + "/")))
 
         # self.prefix is an absolute path, so we cannot append it to another path.
         self.rel_prefix = self.prefix.relative_to(self.prefix.root)