Apple: Set CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG on non-macOS too

Since CMake 3.19, we no longer support macOS SDKs older than 10.5,
which corresponds to Xcode 3. Supporting older Xcode versions for
device platforms is also not realistic. We therefore expect the -rpath
linker option should always be supported now.

When targeting iOS, tvOS or watchOS, the previous disabling of -rpath
support meant that the install_name_dir of shared libraries and
frameworks was unable to use @rpath. This resulted in embedding
absolute paths for their install_name. When they were embedded in an
app bundle, this would cause the app to fail at runtime. By enabling the
-rpath linker option, the default install_name_dir is now @rpath for these platforms, which results in binaries that do work at runtime.

Fixes: #20036

Help/prop_tgt/INSTALL_NAME_DIR.rst

 INSTALL_NAME_DIR
 ----------------
 
-macOS directory name for installed targets.
+Directory name for installed targets on Apple platforms.
 
 ``INSTALL_NAME_DIR`` is a string specifying the directory portion of the
-"install_name" field of shared libraries on macOS to use in the
-installed targets.
+"install_name" field of shared libraries on Apple platforms for
+installed targets.  When not set, the default directory used is determined
+by :prop_tgt:`MACOSX_RPATH`.  Policies :policy:`CMP0068` and :policy:`CMP0042`
+are also relevant.
 
 This property is initialized by the value of the variable
 :variable:`CMAKE_INSTALL_NAME_DIR` if it is set when a target is

Help/release/3.20.rst

@@ -341,3 +341,9 @@ Changes made since CMake 3.20.0 include the following.
   and ``Intl_IS_BUILTIN``, but they were not implemented correctly.
   These have been removed and replaced with a single ``Intl_IS_BUILT_IN``
   check, whose name is consistent with the :module:`FindIconv` module.
+
+* The ``-rpath`` linker flag is now specified as supported on all Apple
+  platforms, not just macOS.  The ``install_name_dir`` used for
+  iOS, tvOS and watchOS should now default to ``@rpath`` instead of using
+  a full absolute path and failing at runtime when the library or framework
+  is embedded in an application bundle (see :prop_tgt:`XCODE_EMBED_<type>`).

Help/variable/CMAKE_INSTALL_NAME_DIR.rst

 CMAKE_INSTALL_NAME_DIR
 ----------------------
 
-macOS directory name for installed targets.
+Directory name for installed targets on Apple platforms.
 
 ``CMAKE_INSTALL_NAME_DIR`` is used to initialize the
 :prop_tgt:`INSTALL_NAME_DIR` property on all targets.  See that target

Modules/Platform/Darwin.cmake

@@ -47,11 +47,7 @@ set(CMAKE_SHARED_MODULE_PREFIX "lib")
 set(CMAKE_SHARED_MODULE_SUFFIX ".so")
 set(CMAKE_MODULE_EXISTS 1)
 set(CMAKE_DL_LIBS "")
-
-# Enable rpath support for 10.5 and greater where it is known to work.
-if("${DARWIN_MAJOR_VERSION}" GREATER 8)
-  set(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG "-Wl,-rpath,")
-endif()
+set(CMAKE_SHARED_LIBRARY_RUNTIME_C_FLAG "-Wl,-rpath,")
 
 foreach(lang C CXX OBJC OBJCXX)
   set(CMAKE_${lang}_OSX_COMPATIBILITY_VERSION_FLAG "-compatibility_version ")