cmake: Fix IF_CHECK_PASSED option handling

`IF_CHECK_PASSED` is a multi-value keyword, resulting in a list value.
Convert it to a string before applying any `string()` command.

cmake/module/TryAppendCXXFlags.cmake

@@ -72,7 +72,8 @@ function(try_append_cxx_flags flags)
         target_compile_options(${TACXXF_TARGET} INTERFACE ${TACXXF_IF_CHECK_PASSED})
       endif()
       if(DEFINED TACXXF_VAR)
-        string(STRIP "${${TACXXF_VAR}} ${TACXXF_IF_CHECK_PASSED}" ${TACXXF_VAR})
+        list(JOIN TACXXF_IF_CHECK_PASSED " " flags_if_check_passed_as_string)
+        string(STRIP "${${TACXXF_VAR}} ${flags_if_check_passed_as_string}" ${TACXXF_VAR})
       endif()
     else()
       if(DEFINED TACXXF_TARGET)