Added a build script.

The build script was added to obtain the correct values of the
errno C preprocessor definitions. This way no matter what the platform
has defined them as, they will be correct in the libraries code.

src/c/mod.rs

@@ -3,10 +3,12 @@
 //! this library is as up-to-date as it can be.
 
 
+mod errno;
 mod string;
 mod time;
 
 
 
+pub use self::errno::*;
 pub use self::string::*;
 pub use self::time::*;