Throw an informative error if Qt is not present

[kgrz]

Right now, running "gem install capybara-webkit" on a machine that
doesn't have Qt binaries installed fails with the
message "'qmake -spec macs-g++' not available". This change will check
if a binary is installed and throw a descriptive error.

Fixes #590

[jferris]

Thanks for the pull request. A couple of small things:

• We've tried to keep the code in CapybaraWebkitBuilder. If you could move the check into the first step that looks for qmake, that would be awesome.
• We look for several binaries like qmake-qt4 and allow users to override the binary with the QMAKE environment variable. This change would cause the build to crash on those configurations.

Do you want to make those changes?

[kgrz]

@jferris Thank you so much for your patience :) I made some changes so that the binary-availability checking is not limited to just qmake. I was thinking to extract the raise part into a separate error but the errors inside the lib directory doesn't seem to be the best place to add it and so, I wrote that inline.

[jferris]

@kgrz sorry for the delay here. Looking over your most recent changes, it seems like this will now unconditionally throw an error whenever attempting to build?

[kgrz]

@jferris Apologies :/ I will close this PR and will open a new one with proper code. I don't know why I removed the if conditional at the end and pushed the code.