When executing gem install jekyll on OSX 10.8 with the standalone Command Line Tools package from Apple\'s Developer site installed (no Xcode), i run into the follo         
        
I was having trouble with brew install hg. It was choking on the output of xcodebuild -version.
It is a horrible hack, but I replaced /usr/bin/xcodebuild with a shell script that just echoed "4.3" and was able to install mecurial successfully.
Unfortunately, at least the last time I played with it, I found you really can't use xcrun with just the standalone Command Line Tools package.  It apparently wasn't designed for that use case; the standalone package is a fairly recent innovation with Xcode 4. If the product you are trying to install really depends on xcrun, you may need to install the full Xcode.app distribution to get around it.  That, or modify the distribution's Makefile et al to not use xcrun.  Or, possibly (untested), create some directories and/or symlinks to fake xcrun into thinking you have Xcode.app installed -  a messy hack.
Type in these commands on the console:
sudo xcode-select -switch /usr/bin
sudo mv /usr/bin/xcrun /usr/bin/xcrun-orig
sudo vim /usr/bin/xcrun
Enter the following into your dummy xcrun file:
#!/bin/sh
$@
Then make it executable:
sudo chmod 755 /usr/bin/xcrun
Even with Command Line Tools installed, your xcode-select is strange. It should be pointing into your XCode bundle, not /usr/bin:
$ xcode-select -print-path
/Applications/Xcode.app/Contents/Developer
Switch it to Xcode like this:
$ sudo xcode-select --switch /Applications/Xcode.app