Set Config's *extensions and *_ext vars correctly for Win32 static builds

When building with ALL_STATIC=define on Win32, we arranged for Encode's
sub-extensions to get built (in win32/config_sh.PL), but neglected to list
them in the relevant Config variables, causing test failures in
t/porting/FindExt.t and lib/Config/Extensions.t. Moving the special logic
for Encode into win32/FindExt.pm fixes that.

Fix a related test failure regarding Sys-Syslog (not built on Win32) while
we're at it.