add "Compile a C library while setting custom defines" example

[budziq]

Use cc crate to compile a C library from a build.rs file while setting custom defines/CFLAGS.

Please note that it will be important to mention that this is a build time crate.
Most likely we will dedicate a separate (last) chapter like "build_time.md"

cc #236

[dmfutcher]

I can take a look at this one