windows: initial support for zig build test

Makes progress getting "zig build test" to work on windows.  Mostly
fixed issues around build configuration and added some branches throughout
the Zig code to return/throw errors for unimplemented parts.

I also added an initial implementation for getting the home dir.

pkg/utf8proc/build.zig

@@ -41,6 +41,7 @@ pub fn buildLib(
 
     // Compile
     var flags = std.ArrayList([]const u8).init(b.allocator);
+    try flags.append("-DUTF8PROC_EXPORTS");
     defer flags.deinit();
 
     // C files