only pass clang LTO arguments when they are needed

If the LTO threads == 0 clang will default to the same argument we
manually pass, which meant we dropped support for admittedly ancient
versions of clang that didn't yet add that option.

Drop the extraneous argument, and add a specific error condition when
too old versions of clang are detected.

Fixes #9569

mesonbuild/compilers/mixins/clang.py

@@ -156,7 +156,9 @@ class ClangCompiler(GnuLikeCompiler):
 
     def get_lto_link_args(self, *, threads: int = 0, mode: str = 'default') -> T.List[str]:
         args = self.get_lto_compile_args(threads=threads, mode=mode)
-        # In clang -flto=0 means auto
-        if threads >= 0:
+        # In clang -flto-jobs=0 means auto, and is the default if unspecified, just like in meson
+        if threads > 0:
+            if not mesonlib.version_compare(self.version, '>=4.0.0'):
+                raise mesonlib.MesonException('clang support for LTO threads requires clang >=4.0')
             args.append(f'-flto-jobs={threads}')
         return args