pretty-print build considered harmful

The practice of hiding command execution during the build process frustrates debugging and provides no meaningful benefit. A quick survey of existing GH issues shows several incidents in which submitters who provided build output were asked to take another lap with V=1. We should save everyone a lot of time -- including people who are debugging the build themselves in development or automated build environments -- and just make that behavior the default. If for some reason there is someone relying on not having any usable information about the build and wants the existing behavior, it could be preserved if a similar make variable is set (e.g., SILENT_BUILD=1 or similar).

[isaacs]

@wesolows As a temporary workaround (which of course doesn't help tracking down the bugs we see), you can export V=1 on your own machine.