build: convert parameter to cached value

This converts the local variable to a cached variable which the user can
specify.  By making this a cached variable, it is easier to control and
ensure that a default value is provided.

Author: compnerd

cmake/modules/SwiftConfigureSDK.cmake

@@ -252,9 +252,8 @@ macro(configure_sdk_unix name architectures)
           "${SWIFT_ANDROID_NDK_PATH}/toolchains/${SWIFT_SDK_ANDROID_ARCH_${arch}_NDK_TRIPLE}-${SWIFT_ANDROID_NDK_GCC_VERSION}/prebuilt/${_swift_android_prebuilt_build}")
       endif()
     else()
-      if(NOT SWIFT_SDK_${prefix}_ARCH_${arch}_PATH)
-        set(SWIFT_SDK_${prefix}_ARCH_${arch}_PATH "/")
-      endif()
+      set(SWIFT_SDK_${prefix}_ARCH_${arch}_PATH "/" CACHE STRING "CMAKE_SYSROOT for ${prefix} ${arch}")
+
       if("${prefix}" STREQUAL "HAIKU")
         set(SWIFT_SDK_HAIKU_ARCH_${arch}_LIBC_INCLUDE_DIRECTORY "/system/develop/headers/posix" CACHE STRING "Path to C library headers")
         set(SWIFT_SDK_HAIKU_ARCH_${arch}_LIBC_ARCHITECTURE_INCLUDE_DIRECTORY "/system/develop/headers" CACHE STRING "Path to C library architecture headers")