Bootstrap support

[edenhill]

It would be sweet, neat and practical if mklove could provide rudimentary bootstrapping
of required dependencies.

I.e., if a dependency check fails during configure and the dependency is specified with enough information (url, build command, etc), mklove can automatically download, build and install the dependency during the configure phase and then re-run the failing check.

This should probably be disabled by default (but that default would be changable by configure.<app>),
and enabled through ./configure --bootstrap