Problem in installing X11/XQuartz in Mac 10.8.5
You may be able to install the software using installer on the command line. You will need to make sure that you have the XQuartz disk image mounted before performing this command:

sudo installer -pkg /Volumes/XQuartz-2.7.4/XQuartz.pkg -target /


Edit

Apparently, the XQuartz installer is not allowing installation on the boot drive because it is failing some pre-installation checks. This is based on the following error message that occurred when attempting to use installer in Terminal:

installer: Cannot install on volume / because it is disabled.
 installer: This update requires you to first update your system to OS-X 10.6.3


The installer is not recognizing the system as the correct version. Although the contents of /System/Library/CoreServices/SystemVersion.plist shows that the value of ProductVersion is set to '10.8.5', the installer does not see it this way.

It may be possible to modify the package's Distribution file to allow it to install correctly:


Mount the XQuartz-2.7.4.dmg disk image (assuming that it's on the Desktop):

hdiutil mount ~/Desktop/XQuartz-2.7.4.dmg
Expand the package into a folder name XQuartz on the Desktop:

sudo pkgutil --expand /Volumes/XQuartz-2.7.4/XQuartz.pkg ~/Desktop/XQuartz
Modify the Distribution file inside of the expanded package, to allow installation to occur (regardless of the system version):

cat ~/Desktop/XQuartz/Distribution | sed s/false/true/g &gt; ~/Desktop/Distribution
Move the new Distribution file into the expanded package

sudo mv ~/Desktop/Distribution ~/Desktop/XQuartz
Flatten the modified package:

sudo pkgutil --flatten ~/Desktop/XQuartz ~/Desktop/XQuartz.pkg
Cleanup the XQuartz folder that was expanded from the original package

sudo rm -rf ~/Desktop/XQuartz


After creating the modified package on the Desktop, you may be able to double-click on the file to get it to install normally. Alternately, installing using the Terminal may also work at this point.