This is not the best default value because it imposes additional
requirements on user builds that are not strictly necessary. This
has caused many complaints in the past as people encounter build
failures that are merely due to warnings and not hard compiler
errors.
Changing this default value emakes it more likely that someone trying
to use SFML can use it without issue.
The use of target_compile_options makes it easy to append warnings
to a target. The use of generator expressions also more succinctly
handles a few edge cases for compiler bugs and platform-specific
oddities.
This should be easy to read and maintain than the variable-based
solution it replaces.
