A+C: Ahmed Waheed Moanes (individual CLA)

Generated by a+c.

R=gobot
CC=golang-codereviews
https://codereview.appspot.com/138550043
cmd/go: use pkg-config include pathes in swig and don't double compile c++ files.

Fixes issue 8566.

LGTM=iant
R=iant
CC=golang-codereviews
https://codereview.appspot.com/126210045

Committer: Ian Lance Taylor <iant@golang.org>