[Build] Add SWIFT_SYSTEM_NAME setting.

This lets us override the library subdir name when its derivation
isn't straightforwardly just the lower-case version of `CMAKE_SYSTEM_NAME`.

rdar://123381867

Author: al45tair

CMakeLists.txt

@@ -13,6 +13,14 @@ project(Foundation
   LANGUAGES C Swift)
 enable_testing()
 
+if(NOT SWIFT_SYSTEM_NAME)
+  if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
+    set(SWIFT_SYSTEM_NAME macosx)
+  else()
+    set(SWIFT_SYSTEM_NAME "$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>")
+  endif()
+endif()
+
 # NOTE(compnerd) default to /MD or /MDd by default based on the configuration.
 # Cache the variable to allow the user to alter the configuration.
 set(CMAKE_MSVC_RUNTIME_LIBRARY "MultiThreaded$<$<CONFIG:Debug>:Debug>DLL" CACHE
@@ -99,13 +107,13 @@ if(NOT BUILD_SHARED_LIBS)
   endif()
 
   install(TARGETS CoreFoundation CFXMLInterface
-    DESTINATION lib/swift_static/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>)
+    DESTINATION lib/swift_static/${SWIFT_SYSTEM_NAME})
 
   if(BUILD_NETWORKING)
     set_property(GLOBAL APPEND PROPERTY Foundation_EXPORTS
       CFURLSessionInterface)
     install(TARGETS CFURLSessionInterface
-      DESTINATION lib/swift_static/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>)
+      DESTINATION lib/swift_static/${SWIFT_SYSTEM_NAME})
   endif()
 endif()
 

Sources/BlocksRuntime/CMakeLists.txt

@@ -11,5 +11,5 @@ set_target_properties(BlocksRuntime PROPERTIES
 add_library(BlocksRuntime::BlocksRuntime ALIAS BlocksRuntime)
 
 install(TARGETS BlocksRuntime
-  ARCHIVE DESTINATION lib/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>
-  LIBRARY DESTINATION lib/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>)
+  ARCHIVE DESTINATION lib/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/${SWIFT_SYSTEM_NAME}
+  LIBRARY DESTINATION lib/swift$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:_static>/${SWIFT_SYSTEM_NAME})

Sources/Tools/plutil/CMakeLists.txt

@@ -28,7 +28,7 @@ if(NOT CMAKE_SYSTEM_NAME MATCHES "Darwin|Windows")
 endif()
 
 set_target_properties(plutil PROPERTIES
-  INSTALL_RPATH "$ORIGIN/../lib/swift/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>")
+  INSTALL_RPATH "$ORIGIN/../lib/swift/${SWIFT_SYSTEM_NAME}")
 
 
 set_property(GLOBAL APPEND PROPERTY Foundation_EXPORTS plutil)

Sources/UUID/CMakeLists.txt

@@ -28,7 +28,7 @@ if(NOT BUILD_SHARED_LIBS)
   # TODO(drexin): should be installed in architecture specific folder, once
   # the layout is fixed for non-Darwin platforms
   install(TARGETS uuid
-    ARCHIVE DESTINATION lib/swift_static/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>
-    LIBRARY DESTINATION lib/swift_static/$<LOWER_CASE:${CMAKE_SYSTEM_NAME}>
+    ARCHIVE DESTINATION lib/swift_static/${SWIFT_SYSTEM_NAME}
+    LIBRARY DESTINATION lib/swift_static/${SWIFT_SYSTEM_NAME}
     RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR})
 endif()

cmake/modules/SwiftSupport.cmake

@@ -49,12 +49,7 @@ endfunction()
 # Sets ${result_var_name} with the converted OS name derived from
 # CMAKE_SYSTEM_NAME.
 function(get_swift_host_os result_var_name)
-  if(CMAKE_SYSTEM_NAME STREQUAL Darwin)
-    set(${result_var_name} macosx PARENT_SCOPE)
-  else()
-    string(TOLOWER ${CMAKE_SYSTEM_NAME} cmake_system_name_lc)
-    set(${result_var_name} ${cmake_system_name_lc} PARENT_SCOPE)
-  endif()
+  set(${result_var_name} ${SWIFT_SYSTEM_NAME} PARENT_SCOPE)
 endfunction()
 
 function(_install_target module)