Felix Schwitzer authored Mar 26, 2015 and Brad King committed Mar 30, 2015

Add the dependency on the main swig input source file as a normal
DEPENDS option.  We cannot use MAIN_DEPENDENCY because if there are
multiple target languages then multiple custom commands would want to
use the same MAIN_DEPENDENCY, but at most one custom command may specify
a given source file as its MAIN_DEPENDENCY.  Exposed by a CMP0057
warning.