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/variable/CMAKE_OSX_ARCHITECTURES.rst

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+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

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+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

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+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

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+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.