build: Save configure options and re-use them for automatic runs of configure

This supports the case of a user running "configure --prefix=/foo" then later
updating the soruce (including the configure script) and re-running make.

In this case, the make invocation will re-run configure. Before this change,
this run of configure would lose the user's carefully chosen prefix. This
is now fixed so that configrue is re-run with the user's options.