Ensure meson's prefix is a valid absolute path (#17206)

The path / is not considered an absolute path on Windows, failing with:

    ERROR: prefix value '/' must be an absolute path

os.path.abspath ensures that "/" becomes a valid absolute path on all major
platforms, resolving to "/" on Linux and macOS, and to "C:/" (or whichever
filesystem is currently active, could be E:/ as well) on Windows.

Closes #17204.

conan/tools/meson/meson.py

@@ -46,8 +46,7 @@ def configure(self, reconfigure=False):
             else:  # extra native file for cross-building scenarios
                 cmd += f' --native-file "{native}"'
         cmd += ' "{}" "{}"'.format(build_folder, source_folder)
-        # Issue related: https://github.com/mesonbuild/meson/issues/12880
-        cmd += ' --prefix=/'  # this must be an absolute path, otherwise, meson complains
+        cmd += f" --prefix={self._prefix}"
         self._conanfile.output.info("Meson configure cmd: {}".format(cmd))
         self._conanfile.run(cmd)
 
@@ -112,3 +111,30 @@ def _install_verbosity(self):
         # so it's a bit backwards
         verbosity = self._conanfile.conf.get("tools.build:verbosity", choices=("quiet", "verbose"))
         return "--quiet" if verbosity else ""
+
+    @property
+    def _prefix(self):
+        """Generate a valid ``--prefix`` argument value for meson.
+        For conan, the prefix must be similar to the Unix root directory ``/``.
+
+        The result of this function should be passed to
+        ``meson setup --prefix={self._prefix} ...``
+
+        Python 3.13 changed the semantics of ``/`` on the Windows ntpath module,
+        it is now special-cased as a relative directory.
+        Thus, ``os.path.isabs("/")`` is true on Linux but false on Windows.
+        So for Windows, an equivalent path is ``C:\\``. However, this can be
+        parsed wrongly in meson in specific circumstances due to the trailing
+        backslash. Hence, we also use forward slashes for Windows, leaving us
+        with ``C:/`` or similar paths.
+
+        See also
+        --------
+        * The meson issue discussing the need to set ``--prefix`` to ``/``:
+            `mesonbuild/meson#12880 <https://github.com/mesonbuild/meson/issues/12880>`_
+        * The cpython PR introducing the ``/`` behavior change:
+            `python/cpython#113829 <https://github.com/python/cpython/pull/113829>`_
+        * The issue detailing the erroneous parsing of ``\\``:
+            `conan-io/conan#14213 <https://github.com/conan-io/conan/issues/14213>`_
+        """
+        return os.path.abspath("/").replace("\\", "/")