font-use-system-font

font-use-system-font is a variable defined in `C source code'.
Its value is

nil

Documentation:
Non-nil means to apply the system defined font dynamically.
When this is non-nil and the system defined fixed width font changes, we
update frames dynamically.
If this variable is nil, Emacs ignores system font changes.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.