Update manual_cpp: convert dashes in tmp_filename to underscores

tmp_filename is cropped to the last 250 characters. If the first character after cropping happens do be a dash the compiler will interpret it as a flag and not the argument passed to `-o` resulting in a compilation failure (missing arguments).

manual_cpp

@@ -70,6 +70,7 @@ try:
 except ValueError:
     pass
 
+tmp_filename = tmp_filename.replace("-", "_")
 tmp_filename = tmp_filename[-250:]
 
 if (ext==".cu"):