Variable: x-use-underline-position-properties

Non-nil means make use of UNDERLINE_POSITION font properties.
A value of nil means ignore them. If you encounter fonts with bogus
UNDERLINE_POSITION font properties, for example 7x13 on XFree prior
to 4.1, set this to nil. You can also use `underline-minimum-offset'
to override the font's UNDERLINE_POSITION for small font display sizes.