makefile improvements

'make dependencies' would fail after upgrading go if you did not reinstall the standard library. This hsa been fixed by adding 'go install ./...' to the head of the 'make dependencies' command, which slows things down by a tiny fraction but prevents a class of errors which may frustrate users.

The '-a' flag has been removed from the makefile, because as of a recent version of go it is no longer necessary to have that flag in order to have up-to-date standard library packages. Further, having the '-a' flag would cause permission issues on systems where $GOROOT or $GOPATH was owned by root. Removing the flag removes the problem.