%*CUSTOM_LIB needs to be in @*INC, too.

Not that @*INC actually works in Niecza, but this patch has the same effect.

src/niecza

@@ -79,7 +79,7 @@ EOM
 my $runobj = Q:CgOp { (box Str (cb_get_basedir)) };
 my $basedir = $runobj.IO.append("..").realpath;
 
-my @lib  = $basedir.append("lib"), ".".IO.realpath;
+my @lib  = $basedir.append("lib"), ".".IO.realpath, (Q:CgOp { (sysquery (i 7)) } ~ "/niecza").IO.realpath;
 my $lang = "CORE";
 my $safe = False;
 my $bcnd = "dotnet";