build_and_install.sh: Offer fix when xcodebuild can't find Xcode.app.

If the Xcode command line tools were set to use a "command line tools
instance", which can be installed without having Xcode installed,
build_and_install.sh would fail. It prints an error message with a
command that can fix it if you do have Xcode installed, but the message
was kind of confusing and the command would fail if you didn't run it as
root.

build_and_install.sh now offers to run the command for you and then
continues the installation. I've also tried to make the message a bit
clearer and cleaned up some of the code.

Also fixes another bug that occurred with this configuration problem,
where the error from xcodebuild would be printed at an unintended (and
confusing) point in the script.

Fixes #108.