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