- Don't call both AC_CHECK_FUNCS and AC_REPLACE_FUNC for 'hypot', as the
latter already does everything the former does (because it's implemented as
a call to the former.)

- Don't call AC_CHECK_FUNC() without any 'action' clauses or with an action
clause that just defines HAVE_<function>. Instead, call AC_CHECK_FUNCS,
which defines 'HAVE_<function>' of itself, possibly with aditional 'action'
clauses.

No checks are removed by this patch, only moved around, and some duplicates
are removed.