Split the makefile php, mysql, and nginx builds.

It's sometimes useful to be able to do the build and install
procedures separately (something make people figured out a long
time ago with configure->make->make install).  For instance, when
updating nginx on a live server, you want to leave the old
server running as long as possible before you shut it down to replace
it.

Breaking php, nginx, and mysql's build procedures into discrete
build and install steps makes this makefile more useful when doing
things other than building a local dev machine.