Add Rugged cross compilation support.
Fat gem support for windows.
Update to the latest rake-compiler (pre-)release.
Ignore the pkg dir.
Ignore rugged binaries in lib and subfolders.
Allow specifying the path to libgit2, but fallback to the included version.
Fix an issue with ENV vars I had when running the 'sh' helper on 1.9.3.
Rename embedded_clean to clean:libgit2.
Strip binaries when cross-compiling.
Always prefer the bundled libgit2 version, unless explicitly specified otherwise.

preserving_globas is stolen from the Nokogiri extconf.rb.
Can travis cross compile?
Travis?
Always use the vendored libgit2, unless something else is specified.
Allow to cross compile x64 binaries.
Make sure we use the correct `strip` binary.
Compiling libgit2 should be a pre-requisite of the rugged binary.

Previously, the libgit2 binary would not be built when calling the
`rake cross native gem` tasks.
Make sure we always remove libgit2 build artifacts.

The `clean:libgit2` task was only executed once even when building for
multiple platforms, which would fail.
Fix the path the libgit2 static library is copied to.
Updated to latest rake-compiler.
Get the host platform based on the gcc binary name.