build: correct the OS directory on install

Convert the OS name to lower case as is the convention.  This installs
the content to the right location to permit use from an installed
package.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index e83a858..6eac10b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -131,11 +131,13 @@
                     XCTest
                   USES_TERMINAL)
 
+string(TOLOWER ${CMAKE_SYSTEM_NAME} SWIFT_OS)
+
 install(FILES
           ${CMAKE_CURRENT_BINARY_DIR}/swift/XCTest.swiftdoc
           ${CMAKE_CURRENT_BINARY_DIR}/swift/XCTest.swiftmodule
         DESTINATION
-          ${CMAKE_INSTALL_FULL_LIBDIR}/swift/${CMAKE_SYSTEM_NAME}/${CMAKE_SYSTEM_PROCESSOR})
+          ${CMAKE_INSTALL_FULL_LIBDIR}/swift/${SWIFT_OS}/${CMAKE_SYSTEM_PROCESSOR})
 install(FILES
           ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_SHARED_LIBRARY_PREFIX}XCTest${CMAKE_SHARED_LIBRARY_SUFFIX}
         DESTINATION