blt_append_custom_compiler_flag macro should support list parameters

Nobody has claimed this yet.

Our flag related macros have recently been unified to operate on lists (#305),
but our macro to generate flags -- ``blt_append_custom_compiler_flag`` -- only operates on strings, and the results are not directly compatible with our flag-related macros.

E.g. I'd expect the following to work:
```cmake
blt_append_custom_compiler_flag(FLAGS_VAR _flags
DEFAULT " "
GNU "string-flag1 string-flag2" # space-delimited
CLANG list-flag1 list-flag2 # list
MSVC single )
blt_add_target_compile_flags(TO SCOPE FLAGS ${_flags})
```
But, since the arguments to ``blt_append_custom_compiler_flag`` are strings rather than list (i.e. ``singleValuedArgs`` in our macro, rather than ``multiValuedArgs``), only the first flag is used.

Note: I was able to get my example working (after some effort), but it is not as simple or obvious as it can be:

```cmake
# Current workaround
blt_append_custom_compiler_flag(FLAGS_VAR _flags
DEFAULT " "
GNU "string-flag1 string-flag2"
CLANG "string-flag3 string-flag4"
MSVC single )
string (REPLACE " " ";" _flags "${_flags}")
blt_add_target_compile_flags(TO SCOPE FLAGS ${_flags})
```
Note: This only works for space-delimited strings and requires a manual conversion from strings to lists.

Locate the blt_append_custom_compiler_flag macro and the flag-related macro tests or examples. Start by tracing how GNU, CLANG, MSVC, and DEFAULT arguments are parsed, then verify that space-delimited strings and list-valued arguments produce flags usable by blt_add_target_compile_flags; done means the manual string-to-list conversion is no longer required.

Written by the indexing model from the issue text.