Updating Manifest.txt.

Related to f75a283 (#495).
MiniPortile checkpoints during cross-compile include host.

Preparing for more MiniPortile builds.
Using mini_portile to compile libxml2 and libxslt on any (non-windows) platform.
Be verbose when cross compilation is turned off.
MiniPortile recipes global now uses strings instead of symbols for keys.

Preparation for tidying up dependency declarations.
dependencies.yml is now the central location for library dependencies.

Note that this is for both windows cross-compile and packaged dependencies.
Upgrading libxml2 dependency from 2.7.7 to 2.7.8.

Affects only windows users at this time.
mini_portile is a gem dependency if we're building libxml2.
Extracting the "add to gem manifest dynamically" functionality.
If we're building libxml2, include the tarballs in the gem manifest.