Don't wipe out RPATHs specified by dependencies

Since we scan all dependencies for build-only RPATHs now (which are
removed on install), we must take care not to add build-only RPATHs
pointing to directories that dependencies explicitly add -Wl,-rpath
link args for, otherwise the paths will get wiped on install.

Caught by LinuxlikeTests::test_usage_pkgconfig_prefixes

mesonbuild/backend/backends.py

@@ -708,7 +708,6 @@ def _libdir_is_system(libdir: str, compilers: T.Mapping[str, 'Compiler'], env: '
         return False
 
     def get_external_rpath_dirs(self, target: build.BuildTarget) -> T.Set[str]:
-        dirs: T.Set[str] = set()
         args: T.List[str] = []
         for lang in LANGUAGES_USING_LDFLAGS:
             try:
@@ -719,6 +718,11 @@ def get_external_rpath_dirs(self, target: build.BuildTarget) -> T.Set[str]:
                     args.extend(e)
             except Exception:
                 pass
+        return self.get_rpath_dirs_from_link_args(args)
+
+    @staticmethod
+    def get_rpath_dirs_from_link_args(args: T.List[str]) -> T.Set[str]:
+        dirs: T.Set[str] = set()
         # Match rpath formats:
         # -Wl,-rpath=
         # -Wl,-rpath,
@@ -777,6 +781,8 @@ def rpaths_for_non_system_absolute_shared_libraries(self, target: build.BuildTar
                     paths.add(os.path.join(self.build_to_src, rel_to_src))
                 else:
                     paths.add(libdir)
+            # Don't remove rpaths specified by the dependency
+            paths.difference_update(self.get_rpath_dirs_from_link_args(dep.link_args))
         for i in chain(target.link_targets, target.link_whole_targets):
             if isinstance(i, build.BuildTarget):
                 paths.update(self.rpaths_for_non_system_absolute_shared_libraries(i, exclude_system))

mesonbuild/mesonlib/universal.py

@@ -1756,6 +1756,10 @@ def update(self, iterable: T.Iterable[_T]) -> None:
     def difference(self, set_: T.Union[T.Set[_T], 'OrderedSet[_T]']) -> 'OrderedSet[_T]':
         return type(self)(e for e in self if e not in set_)
 
+    def difference_update(self, iterable: T.Iterable[_T]) -> None:
+        for item in iterable:
+            self.discard(item)
+
 def relpath(path: str, start: str) -> str:
     # On Windows a relative path can't be evaluated for paths on two different
     # drives (i.e. c:\foo and f:\bar).  The only thing left to do is to use the