[Build] Use SWIFT_SYSTEM_NAME rather than just lowercasing.

Lowercasing `CMAKE_SYSTEM_NAME` to find the name Swift uses for a platform
is not guaranteed to work.  Instead, use the `SWIFT_SYSTEM_NAME` variable,
which lets us override the name when building the Static SDK for Linux.

Fixes apple#420.

CMakeLists.txt

@@ -15,6 +15,14 @@ cmake_minimum_required(VERSION 3.16)
 project(SwiftCollections
  LANGUAGES C Swift)
 
+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()
+
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake/modules)
 
 set(CMAKE_Swift_MODULE_DIRECTORY ${CMAKE_BINARY_DIR}/swift)

cmake/modules/SwiftSupport.cmake

@@ -59,12 +59,7 @@ 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)