Enable Support for Cross Compilation

[GreenYun]

I have suffering cross-compiling lvgl and lvgl-sys as dependencies of my project since the invocation of bindgen would always using my host toolchain.

I cannot find any better idea than rewriting the build script to add more environment variables and compiler flags.

Issue #53 gave ideas using the cross tool that is not suitable for my environment. In the situation of my work, I have no Docker environment even the toolchain is not provided by any system package manager. I have only a directory with pre-compiled Xcompiler. For normal rust projects, specifying target triples and providing linkers to rustc will get everything worked.

I wonder if there are any workarounds to provide enough information to the build system and get things worked?