Makefile should use -installsuffifx in addition to -netgo

[bboreham]

After installing a fresh copy of Go, attempting to build weave gives:

Your go standard library was built without the 'netgo' build tag.
To fix that, run
    sudo go clean -i net
    sudo go install -tags netgo std

These instructions overwrite Go's libraries, which may be inconvenient for some users.
This can be improved by adding -installsuffix netgo each time -tags netgo is used in the Makefile to make Go put the files in a different directory.
(Inspired by golang/go#9369 (comment))

[bboreham]

Since this issue was filed, the symptoms have downgraded to:
After installing a fresh copy of Go, attempting to build weave gives:

$ make
go get -tags netgo ./prog/weaver
go install net: open /usr/local/go/pkg/linux_amd64/net.a: permission denied
make: *** [prog/weaver/weaver] Error 1