Since build.rs requires MagickWand-config, make an effort to determine
if the executable is available or not, and panic with a clear course of
action. Otherwise the error is rather cryptic.
Fixes#3
cargo test passes
First, only add the special environment variable when building on Mac OS
X. Second, look for the path to ensure the Xcode command line tools are
installed, and panic if that is not the case.
cargo test passes
Leave the generated bindings out of the source repository and instead
have Cargo automatically build them if they are missing. This allows the
version of MagickWand to be incorporated into the bindings
automatically. For instance, in Homebrew the version is 6.Q16, while on
FreeBSD 10.2 it is 6.9, and only be generating the bindings for each
system can we ensure a smooth compilation process.
