diff --git a/llvm/cmake/config-ix.cmake b/llvm/cmake/config-ix.cmake
--- a/llvm/cmake/config-ix.cmake
+++ b/llvm/cmake/config-ix.cmake
@@ -692,6 +692,17 @@
 endif()
 
 function(llvm_get_host_prefixes_and_suffixes)
+  # Not all platform files will set these variables (relying on them being
+  # implicitly empty if they're unset), so unset the variables before including
+  # the platform file, to prevent any values from the target system leaking.
+  unset(CMAKE_STATIC_LIBRARY_PREFIX)
+  unset(CMAKE_STATIC_LIBRARY_SUFFIX)
+  unset(CMAKE_SHARED_LIBRARY_PREFIX)
+  unset(CMAKE_SHARED_LIBRARY_SUFFIX)
+  unset(CMAKE_IMPORT_LIBRARY_PREFIX)
+  unset(CMAKE_IMPORT_LIBRARY_SUFFIX)
+  unset(CMAKE_EXECUTABLE_SUFFIX)
+  unset(CMAKE_LINK_LIBRARY_SUFFIX)
   include(Platform/${CMAKE_HOST_SYSTEM_NAME} OPTIONAL RESULT_VARIABLE _includedFile)
   if (_includedFile)
     set(LLVM_HOST_STATIC_LIBRARY_PREFIX ${CMAKE_STATIC_LIBRARY_PREFIX} PARENT_SCOPE)