OS X: Add CMP0042 to enable MACOSX_RPATH by default

Also adding documentation for CMAKE_MACOSX_RPATH, and improving
documentation for MACOSX_RPATH.

Help/manual/cmake-policies.7.rst

@@ -75,3 +75,4 @@ All Policies
    /policy/CMP0039
    /policy/CMP0040
    /policy/CMP0041
+   /policy/CMP0042

Help/manual/cmake-variables.7.rst

@@ -207,6 +207,7 @@ Variables that Control the Build
    /variable/CMAKE_LINK_LIBRARY_FILE_FLAG
    /variable/CMAKE_LINK_LIBRARY_FLAG
    /variable/CMAKE_MACOSX_BUNDLE
+   /variable/CMAKE_MACOSX_RPATH
    /variable/CMAKE_MAP_IMPORTED_CONFIG_CONFIG
    /variable/CMAKE_MODULE_LINKER_FLAGS_CONFIG
    /variable/CMAKE_MODULE_LINKER_FLAGS

Help/policy/CMP0042.rst

@@ -0,0 +1,19 @@
+CMP0042
+-------
+
+:prop_tgt:`MACOSX_RPATH` is enabled by default.
+
+CMake 2.8.12 and newer has support for using ``@rpath`` in a target's install
+name.  This was enabled by setting the target property
+:prop_tgt:`MACOSX_RPATH`.  The ``@rpath`` in an install name is a more
+flexible and powerful mechanism than ``@executable_path`` or ``@loader_path``
+for locating shared libraries.
+
+CMake 3.0.0 and later prefer this property to be ON by default.  Projects
+wanting ``@rpath`` in a target's install name may remove any setting of
+the :prop_tgt:`INSTALL_NAME_DIR` and :variable:`CMAKE_INSTALL_NAME_DIR`
+variables.
+
+This policy was introduced in CMake version 3.0.0.  CMake version
+|release| warns when the policy is not set and uses OLD behavior.  Use
+the cmake_policy command to set it to OLD or NEW explicitly.

Help/prop_tgt/MACOSX_RPATH.rst

@@ -4,7 +4,15 @@ MACOSX_RPATH
 Whether to use rpaths on Mac OS X.
 
 When this property is set to true, the directory portion of
-the"install_name" field of shared libraries will default to
-"@rpath".Runtime paths will also be embedded in binaries using this
-target.This property is initialized by the value of the variable
-CMAKE_MACOSX_RPATH if it is set when a target is created.
+the "install_name" field of shared libraries will be ``@rpath``
+unless overridden by :prop_tgt:`INSTALL_NAME_DIR`.  Runtime
+paths will also be embedded in binaries using this target and
+can be controlled by the :prop_tgt:`INSTALL_RPATH` target property.
+This property is initialized by the value of the variable
+:variable:`CMAKE_MACOSX_RPATH` if it is set when a target is
+created.
+
+Policy CMP0042 was introduced to change the default value of
+MACOSX_RPATH to ON.  This is because use of ``@rpath`` is a
+more flexible and powerful alternative to ``@executable_path`` and
+``@loader_path``.

Help/variable/CMAKE_MACOSX_RPATH.rst

@@ -0,0 +1,7 @@
+CMAKE_MACOSX_RPATH
+-------------------
+
+Whether to use rpaths on Mac OS X.
+
+This variable is used to initialize the :prop_tgt:`MACOSX_RPATH` property on
+all targets.