Remove PKG variable from Makefile.am, as it's redundant with EXENAME.

Also simplify binary targets and allow additional flags to be passed
to `go build` and `go test`.
Allow configuration of the location of vendor Go code.

Also elimate redundant setting of GOPATH for `go` execution and
simplify the `getupdate` target.
Organize all settable Makefile parameters into a single block.
Don't create config.h, it's not needed for a golang only project.
Breaking change: use VENDOR_GOPATH detected by configure for all builds.

This eliminates use of the Debian specific value for SYSTEM_GOPATH and
more readily allows release builds to be made against any set of depenencies.

Additionally, the now redundant `releasetest` target has been removed.
Remove needless AC_SUBST invokations.
Recursively clean all built objects.
Merge pull request #117 from deathwish/improve-makefile

Improve makefile