Add cross compilation for x64-mingw32.
Merge branch 'x64-mingw32' of git://github.com/larskanis/sqlite3-ruby into larskanis-x64-mingw32

Conflicts:
	tasks/vendor_sqlite3.rake
HACK: Expose Ruby C extension task

Make 'sqlite3_native' compilation task available as constant so other
tasks can lookup into definitions like `cross_compile` and configure
options before triggering the build.

This is of course a massive hack around ExtensionTask and Rake, but at
this time there is no better alternative :cry:
WiP: attempt to cross-compile everything

Compiling sqlite3 is becoming a challenge using mini_portile and
rake-compiler.

Testing this inside a Ubuntu VM is resulting into errors about
GCC not been able to find the header files or the library files.

This requires further exploration to work properly.

Most likely: mini_portile will be removed and usage of knapsack
packages will be enforced instead.
Stop messing with LDFLAGS when cross-compiling

LDFLAGS cannot be used to link to specific versions, instead
we should use extconf '--with-opt-dir'.
Return recipe definition for SQLite3

Return the recipe so it can be used to adjust other cross-compilation
options of sqlite3 extension.
Use extconf's '--with-opt-dir' to proper link

On Linux, LDFLAGS cannot be used during cross-compilation and was
resulting in problems.

We use --with-opt-dir to point the right headers/libs location for
that platform.
Retrieve cross-compile host from CC (not host)

Use this to build sqlite3 for the right platform since the generated
triplet for i586-mingw32msvc was incorrect.

Feed the recipe installation location to the cross-configure options
so the Ruby extension finds the proper headers and libs.
Build all the cross ports when 'cross' is invoked

Since 'x64-mingw32' might not get defined on Ruby 1.8.7, we use
this so all the ports get compiled before usage.
Add Gemfile for compat with rake-compiler-dev-box.

Increase version of rake-compiler to 0.9.1. That is needed for the
x64-mingw32 build.

This also solves github issue #53 .
No need to activate the MiniPortile library for cross any longer.

The library paths are passed per --with-opt-dir= to extconf.rb, so there
is no need to set environment variables any longer.

Since 9cfe956 the port tasks are bound to :cross instead of compile:#{platform},
so the environment variables become platform-mixed and could break the build.
Update SQLite port to 3.7.17.
Merge pull request #104 from larskanis/x64-mingw32-lars

More Windows x64 support