more Fix Windows build (of Encode) with VC++ 6.0

more Fix Windows build (of Encode) with VC++ 6.0

The previous commit (080a7046069084) broke g++ builds.
This commit, based on a suggestion by Jan, changes the test of whether
we're "in perl" from being the def'ness of U8 to being the def'ness of
H_Perl; and removes all the '#define U8 U8's that were formerly used to
signal this state.