docs: update CMake documentation to give more guidance (#859)

Update the documentation and indicate how to configure when developing
X10 and when not developing X10.

Co-authored-by: Michelle Casbon <michellecasbon@google.com>
Co-authored-by: Michelle Casbon <texasmichelle@users.noreply.github.com>

Author: compnerd

README.md

@@ -118,16 +118,29 @@ will not allow the use of the `-B` option to specific the build tree and
 require that you are in the location of the build tree (and the `-B` option
 and its argument are elided).
 
+To enable CUDA support, run `export TF_NEED_CUDA=1` before the steps below.
+
 If `swiftc` is not in your `PATH`, you must specify the path to it using
 `-D CMAKE_Swift_COMPILER=`.
 
-To enable CUDA support, run `export TF_NEED_CUDA=1` before the steps below.
+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.
 
+```shell
+cmake -B out -D BUILD_X10=YES -G Ninja -S swift-apis
+cmake --build out
 ```
-cmake -G Ninja -B out -S swift-apis
+
+If you are not intending to develop X10, you can reduce the build times by
+using the bundled X10 in the toolchain using
+`-D USE_BUNDLED_X10=YES -D USE_BUNDLED_CTENSORFLOW=YES`:
+
+```shell
+cmake -B out -D BUILD_X10=YES -D USE_BUNDLED_CTENSORFLOW=YES -D USE_BUNDLED_X10=YES -G Ninja -S swift-apis
 cmake --build out
 ```
 
+
 ## Bugs
 
 Please report bugs and feature requests using GitHub issues in this repository.

cmake/modules/FindX10.cmake

@@ -2,7 +2,9 @@
 if(DEFINED CMAKE_Swift_COMPILER)
   get_filename_component(_Swift_TOOLCHAIN ${CMAKE_Swift_COMPILER} DIRECTORY)
   get_filename_component(_Swift_TOOLCHAIN ${_Swift_TOOLCHAIN} DIRECTORY)
-  file(TO_CMAKE_PATH ${_Swift_TOOLCHAIN}/usr/lib/swift _Swift_LIBDIR)
+  get_filename_component(_Swift_TOOLCHAIN ${_Swift_TOOLCHAIN} DIRECTORY)
+  string(TOLOWER ${CMAKE_SYSTEM_NAME} system_lc)
+  file(TO_CMAKE_PATH ${_Swift_TOOLCHAIN}/usr/lib/swift/${system_lc} _Swift_LIBDIR)
 endif()
 
 find_library(X10_LIBRARY