Add rpath entries for all found libraries outside of system libraries.

docs/markdown/snippets/prebuilt.md

@@ -1,9 +1,14 @@
-# Better support for prebuilt shared libraries
+# Better support for shared libraries in non-system paths
 
 Meson has had support for prebuilt object files and static libraries.
-This release adds feature parity to shared libraries. This means
-that e.g. shipping prebuilt libraries as subprojects now can
-be as simple as writing a definition file that looks like this.
+This release adds feature parity to shared libraries that are either
+in non-standard system paths or shipped as part of your project. On
+systems that support rpath, Meson automatically adds rpath entries
+to built targets using manually found external libraries.
+
+This means that e.g. supporting prebuilt libraries shipped with your
+source or provided by subprojects or wrap definitions by writing a
+build file like this:
 
     project('myprebuiltlibrary', 'c')
     

mesonbuild/backend/backends.py

@@ -303,9 +303,12 @@ class Backend:
         for dep in target.external_deps:
             if isinstance(dep, dependencies.ExternalLibrary):
                 la = dep.link_args
-                if len(la) == 1 and la[0].startswith(self.environment.get_source_dir()):
+                if len(la) == 1 and os.path.isabs(la[0]):
                     # The only link argument is an absolute path to a library file.
                     libpath = la[0]
+                    if libpath.startswith(('/usr/lib', '/lib')):
+                        # No point in adding system paths.
+                        continue
                     if os.path.splitext(libpath)[1] not in ['.dll', '.lib', '.so']:
                         continue
                     absdir = os.path.split(libpath)[0]
@@ -321,7 +324,9 @@ class Backend:
             prospective = self.get_target_dir(ld)
             if prospective not in result:
                 result.append(prospective)
-        result += self.rpaths_for_bundled_shared_libraries(target)
+        for rp in self.rpaths_for_bundled_shared_libraries(target):
+            if rp not in result:
+                result += [rp]
         return result
 
     def object_filename_from_source(self, target, source, is_unity):