Remove 'make prerequisites' and update docs accordingly

Makefile

@@ -79,11 +79,7 @@ clean:
 	rm -f $(WEAVER_EXE) $(WEAVEDNS_EXE) $(WEAVER_EXPORT) $(WEAVEDNS_EXPORT) $(WEAVETOOLS_EXPORT)
 	$(SUDO) rm -rf $(WEAVETOOLS_EXES)
 
-prerequisites:
-	$(SUDO) apt-get -y update
-	$(SUDO) apt-get -y install --no-install-recommends build-essential git ca-certificates golang docker.io mercurial libpcap-dev
-
-build: prerequisites
+build:
 	$(SUDO) go clean -i net
 	$(SUDO) go install -tags netgo std
 	$(MAKE) build_weavetools_exes_in_container=

site/building.md

@@ -31,11 +31,15 @@ $ git clone https://$WEAVE $GOPATH/src/$WEAVE
 $ cd $GOPATH/src/$WEAVE
 ```
 
-Several prerequisites are needed to build weave.  To install these, in
-the weave directory do:
+Some prerequisites are needed to build weave.  First, install Docker
+if you haven't already, by following the instructions (on the Docker
+site)[https://docs.docker.com/installation/ubuntulinux/].  Note that
+we recommend using the Docker-maintained `lxc-docker` package, rather
+than the `docker.io` package which contains a very old version.  Then
+install the other prerequisites for building with:
 
 ```bash
-$ make prerequisites
+$ sudo apt-get install build-essential git golang mercurial libpcap-dev
 ```
 
 Then to actually build, simply do: