Fixed dead code warnings

Closes https://github.com/AngryLawyer/rust-sdl2/issues/127
Merge pull request #128 from bvssvni/master

Fixed dead code warnings
fix makefile pkg-config sdl2 detection
Merge pull request #129 from jdeseno/master

fix makefile pkg-config sdl2 detection
Added rust-empty.mk

To build with rust-empty, type

`make gen-lib && make -f rust-empty.mk`
complete cpu api
Merge pull request #132 from jdeseno/cpuinfo

complete cpu api
Merge pull request #131 from bvssvni/master

Added rust-empty.mk
complete clipboard api
Merge pull request #133 from jdeseno/clipboard

complete clipboard api
complete filesystem api
Merge pull request #134 from jdeseno/filesystem

complete filesystem api
Upgrade to latest Rust
Merge pull request #136 from bvssvni/master

Upgrade to latest Rust
Fix osx+pkg-config build with rust-empty.mk file
Merge pull request #137 from jdeseno/master

Fix osx+pkg-config build with rust-empty.mk file
updated makefile targets