Consider not making FMT_USE_USER_DEFINED_LITERALS enabled by default

[merryhime]

This produces a warning under -Wpendantic on GCC 9.1. The feature this depends upon is non-standard. Unfortunately ISO C++ did not adopt the use of string literal operator templates taking a character pack.

[vitaut]

This is somewhat problematic because it will be a breaking change. However, you can disable user-defined literals by defining FMT_USE_USER_DEFINED_LITERALS to 0 when including {fmt}.

[vitaut]

Actually disabling just UDL templates on GCC 9 by default should be fine because it will fallback to runtime checks. Done in a6e8ed1.