Avoid bogus warnings when configuring for Qt6

Avoid bogus warnings when configuring for Qt6

Recent versions of qmake complain loudly when the locale
does not explicitly specify the utf8 codeset. Specifically,
the configure script exports LC_ALL=C and that implies the
ascii codeset. Previously, this was silently changed to utf8,
whereas now we get a warning for each qmake invocation.
So, explicitly set utf8 before calling qmake.