documentation: improve explanation for CMake based builds (#1161)

documentation: improve explanation for CMake based builds

This adds additional documentation on how to avoid building X10 if the
prebuilt option is being used or if it has been built previously
manually.

Add a note about CMake defaulting to building a copy of X10 if one is
not provided.  Additionally, add examples of what the flags should look
like for various cases to avoid building X10 by default with CMake.

Co-authored-by: Dan Zheng <danielzheng@google.com>

Author: compnerd

Documentation/Development.md

@@ -15,6 +15,10 @@ tensor computation in the TensorFlow Swift APIs. This library builds atop of
 the TensorFlow library, using the XLA compiler to perform optimizations over
 the tensor computation.
 
+In the case that a prebuilt option is not available for your platform, note that
+by default the CMake based build will build a copy of X10 if it does not find
+one.
+
 #### (Option 1) Use a prebuilt version of X10
 
 You can use a prebuilt version of the X10 library for building the TensorFlow
@@ -247,23 +251,55 @@ $ swift test -Xcc -I/Library/tensorflow-2.4.0/usr/include -Xlinker -L/Library/te
 
 #### CMake
 
-*Note: CMake is required for building X10 modules.*
-
-In-tree builds are not supported.  
+*Note: In-tree builds are not supported.*
 
 *Note: To enable CUDA support, run `export TF_NEED_CUDA=1` before the steps below.*
 
 *Note: If `swiftc` is not in your `PATH`, you must specify the path to it using
 `-D CMAKE_Swift_COMPILER=`.*
 
-This will build X10 as part of the build.  Ensure that you do not have the
-x10 modules in the toolchain that you are using to develop here.
+By default, CMake will build X10 if it is not informed of where the library is
+available.  If you wish to build X10 as part of the CMake build, ensure that you
+do not have the X10 modules in the toolchain that you are using to develop here
+(more explicitly, use a stock toolchain rather than a TensorFlow toolchain).
 
 ```shell
-cmake -B out -G Ninja -S swift-apis
+cmake -B out -G Ninja -S swift-apis -D CMAKE_BUILD_TYPE=Release
 cmake --build out
 ```
 
+If you have a prebuilt version of the X10 library that you wish to use (i.e. you
+are using one of the prebuilt libraries referenced above or have built it
+yourself above), you can inform CMake where to find the build time components of
+the library (SDK contents) and avoid building X10.
+
+The path to these libraries is not fixed and depends on your machine setup.
+You should substitute the paths with the appropriate values. In the example
+commands below, we assume that the library is packaged in a traditional Unix
+style layout and placed in `/Library/tensorflow-2.4.0`.
+
+Because the library name differs based on the platform, the following examples
+may help identify what the flags should look like for the target that you are
+building for.
+
+macOS:
+
+```shell
+-D X10_LIBRARY=/Library/tensorflow-2.4.0/usr/lib/libx10.dylib -D X10_INCLUDE_DIRS=/Library/tensorflow-2.4.0/usr/include
+```
+
+Windows:
+
+```shell
+-D X10_LIBRARY=/Library/tensorflow-2.4.0/usr/lib/x10.lib -D X10_INCLUDE_DIRS=/Library/tensorflow-2.4.0/usr/include
+```
+
+Other Unix systems (e.g. Linux, BSD, Android, etc):
+
+```shell
+-D X10_LIBRARY=/Library/tensorflow-2.4.0/usr/lib/libx10.so -D X10_INCLUDE_DIRS=/Library/tensorflow-2.4.0/usr/include
+```
+
 To run tests:
 
 *Note: To view failure output, run `export CTEST_OUTPUT_ON_FAILURE=1` before