CONFIGURE: Fix build under et_EE locale

Freetype-config reports a broken include dir
when et_EE locale is used. We set LC_ALL
and LANGUAGE to "C" explicitly like autoconf
does to force consistent behavior.

From ScummVM's b65365e0d314bff91f5fb3c4fbdd3da7944ea7a2
GRIM: Don't overshoot walk nodes. Fix #699
GRIM: Restore last_set only if engine debugflag is set