Help: Document CMAKE_OSX_* variables

Add documentation entries for variables

 CMAKE_OSX_ARCHITECTURES
 CMAKE_OSX_DEPLOYMENT_TARGET
 CMAKE_OSX_SYSROOT

Explain what each does and when/how they should be set.

Help/manual/cmake-variables.7.rst

@@ -214,6 +214,9 @@ Variables that Control the Build
    /variable/CMAKE_MODULE_LINKER_FLAGS
    /variable/CMAKE_NO_BUILTIN_CHRPATH
    /variable/CMAKE_NO_SYSTEM_FROM_IMPORTED
+   /variable/CMAKE_OSX_ARCHITECTURES
+   /variable/CMAKE_OSX_DEPLOYMENT_TARGET
+   /variable/CMAKE_OSX_SYSROOT
    /variable/CMAKE_PDB_OUTPUT_DIRECTORY
    /variable/CMAKE_PDB_OUTPUT_DIRECTORY_CONFIG
    /variable/CMAKE_POSITION_INDEPENDENT_CODE

Help/prop_tgt/OSX_ARCHITECTURES.rst

@@ -3,9 +3,9 @@ OSX_ARCHITECTURES
 
 Target specific architectures for OS X.
 
-The OSX_ARCHITECTURES property sets the target binary architecture for
-targets on OS X.  This property is initialized by the value of the
-variable CMAKE_OSX_ARCHITECTURES if it is set when a target is
-created.  Use OSX_ARCHITECTURES_<CONFIG> to set the binary
-architectures on a per-configuration basis.  <CONFIG> is an upper-case
-name (ex: "OSX_ARCHITECTURES_DEBUG").
+The ``OSX_ARCHITECTURES`` property sets the target binary architecture for
+targets on OS X (``-arch``).  This property is initialized by the value of the
+variable :variable:`CMAKE_OSX_ARCHITECTURES` if it is set when a target is
+created.  Use :prop_tgt:`OSX_ARCHITECTURES_<CONFIG>` to set the binary
+architectures on a per-configuration basis, where ``<CONFIG>`` is an
+upper-case name (e.g. ``OSX_ARCHITECTURES_DEBUG``).

Help/prop_tgt/OSX_ARCHITECTURES_CONFIG.rst

@@ -4,4 +4,4 @@ OSX_ARCHITECTURES_<CONFIG>
 Per-configuration OS X binary architectures for a target.
 
 This property is the configuration-specific version of
-OSX_ARCHITECTURES.
+:prop_tgt:`OSX_ARCHITECTURES`.

Help/variable/CMAKE_OSX_ARCHITECTURES.rst

+CMAKE_OSX_ARCHITECTURES
+-----------------------
+
+Target specific architectures for OS X.
+
+This variable is used to initialize the :prop_tgt:`OSX_ARCHITECTURES`
+property on each target as it is creaed.  See that target property
+for additional information.
+
+.. include:: CMAKE_OSX_VARIABLE.txt

Help/variable/CMAKE_OSX_DEPLOYMENT_TARGET.rst

+CMAKE_OSX_DEPLOYMENT_TARGET
+---------------------------
+
+Specify the minimum version of OS X on which the target binaries are
+to be deployed.  CMake uses this value for the ``-mmacosx-version-min``
+flag and to help choose the default SDK
+(see :variable:`CMAKE_OSX_SYSROOT`).
+
+If not set explicitly the value is initialized by the
+``MACOSX_DEPLOYMENT_TARGET`` environment variable, if set,
+and otherwise computed based on the host platform.
+
+.. include:: CMAKE_OSX_VARIABLE.txt

Help/variable/CMAKE_OSX_SYSROOT.rst

+CMAKE_OSX_SYSROOT
+-----------------
+
+Specify the location or name of the OS X platform SDK to be used.
+CMake uses this value to compute the value of the ``-isysroot`` flag
+or equivalent and to help the ``find_*`` commands locate files in
+the SDK.
+
+If not set explicitly the value is initialized by the ``SDKROOT``
+environment variable, if set, and otherwise computed based on the
+:variable:`CMAKE_OSX_DEPLOYMENT_TARGET` or the host platform.
+
+.. include:: CMAKE_OSX_VARIABLE.txt

Help/variable/CMAKE_OSX_VARIABLE.txt

+The value of this variable should be set prior to the first
+:command:`project` or :command:`enable_language` command invocation
+because it may influence configuration of the toolchain and flags.
+It is intended to be set locally by the user creating a build tree.
+
+This variable is ignored on platforms other than OS X.