Update llvm/cmake/config-ix.cmake

Co-authored-by: Eli Friedman <[email protected]>

Author: abhina-sree

llvm/cmake/config-ix.cmake

@@ -277,7 +277,7 @@ endif()
 if(LLVM_ENABLE_ICONV)
   if (LLVM_ENABLE_ICONV STREQUAL FORCE_ON)
     find_package(Iconv REQUIRED)
-    if (NOT Iconv_FOUND)
+    if (NOT Iconv_FOUND OR NOT Iconv_IS_BUILT_IN)
       message(FATAL_ERROR "Failed to configure iconv, but LLVM_ENABLE_ICONV is FORCE_ON")
     endif()
   else()