Improve pkg-config error reporting

This is a slightly cleaned up version of what I used to debug why pkg-config was failing (as reported in #208), with this change I get this output:

```
$ cargo build
   Compiling link_glfw v0.1.0 (file:///home/normal/rust/working/piston-examples/glfw-rs)
       Fresh semver v0.0.1 (https://github.com/rust-lang/semver#e17191f5)
   Compiling glfw v0.0.1 (file:///home/normal/rust/working/piston-examples/glfw-rs)
src/glfw/ffi/link.rs:27:1: 27:13 error: error returned by `pkg-config`: (exit code: 1)
`pkg-config stdout`: 
`pkg-config stderr`: Package glfw3 was not found in the pkg-config search path.
Perhaps you should add the directory containing `glfw3.pc'
to the PKG_CONFIG_PATH environment variable
No package 'glfw3' found

src/glfw/ffi/link.rs:27 #[link_glfw]
                        ^~~~~~~~~~~~
error: aborting due to previous error
Could not compile `glfw`.

To learn more, run the command again with --verbose.
```

I am not sure that pkg-config ever actually outputs anything to stdout, so that line may be extraneous. It also doesn't change the error message that brownhead reported, so it only partially fixes #208. There may be a better way to do this, this is just what worked for me as a quick fix.