rust: make bindgen invalid Rust target detection conditional on <0.71

In 0.71.0 the bindgen output for invalid rust targets was changed to
"«version» is not a valid Rust target". This breaks the existing
detection.

Further investigation revealed that from 0.71.0 we probably don't
need to do this check; "all*" Rust versions will be accepted as valid
and converted internally to an appropriate target.

Bindgen does not like `-beta` or `-nightly` suffixes, we'll have
to address that separately depending on the outcome of
rust-lang/rust-bindgen#3152.

See-also: rust-lang/rust-bindgen#2993
Signed-off-by: Matt Jolly <kangie@gentoo.zip>

mesonbuild/modules/rust.py

@@ -254,7 +254,13 @@ def bindgen(self, state: ModuleState, args: T.List, kwargs: FuncBindgen) -> Modu
 
         if self._bindgen_bin is None:
             self._bindgen_bin = state.find_program('bindgen', wanted=kwargs['bindgen_version'])
-            if self._bindgen_rust_target is not None:
+            self._bindgen_bin_version = self._bindgen_bin.get_version()
+            # bindgen 0.71 enables more flexible target specification, any* Rust version string
+            # will be accepted as valid and will be automatically converted to an appropriate target
+            # based on internal logic. See rust-lang/rust-bindgen#2993 for more information.
+            # Additionally the 'invalid Rust target' output is changed in bindgen 0.71 and will not
+            # match the conditional below.
+            if self._bindgen_rust_target is not None and self._bindgen_bin_version < '0.71':
                 # ExternalCommand.command's type is bonkers
                 _, _, err = mesonlib.Popen_safe(
                     T.cast('T.List[str]', self._bindgen_bin.get_command()) +