On Windows, don't try to create relative paths across different drives

test/win/generator-output-different-drive/prog.c

Index: test/win/generator-output-different-drive/prog.c
===================================================================
--- test/win/generator-output-different-drive/prog.c	(revision 0)
+++ test/win/generator-output-different-drive/prog.c	(working copy)
@@ -2,6 +2,6 @@
 
 int main(int argc, char *argv[])
 {
-  printf("Hello from prog1.c\n");
+  printf("Hello from prog.c\n");
   return 0;
 }