Use swig if available, but package wiringpi_wrap.c in the source dist.
diff --git a/MANIFEST.in b/MANIFEST.in
index b146ec8..0c28801 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -4,3 +4,5 @@
 include bindings.i
 include constants.py
 include wiringpi-class.py
+include wiringpi.i
+include wiringpi_wrap.c